Suppose that at the end of day $n$ (after all egg-laying and all possible exchanging for that day) we have $g_n$ geese and $e_n$ eggs, for $n\in\{0,1,2,...\}$. Then the evolution of $\{(g_n,e_n)\}_{n\in\mathbb{N}}$ is given by the following recursion:
$$\begin{cases}
g_{n+1}=g_n+\left\lfloor{e_n+g_n\over 100}\right\rfloor\\[2ex]
e_{n+1}=(e_n+g_n)\bmod 100,\end{cases}\tag{1}$$
where $\lfloor\cdot\rfloor$ and $\bmod$ are the floor function and modulo operator, respectively.
The number of days required to obtain $g'$ geese starting with just $g_0$ geese (and no eggs), can then be expressed as follows:
$$N(g_0,g')=\text{ least }n:\ g_n\ge g',\ e_0=0.\tag{2}$$
NB: Because $g_{n+1}$ depends on both $g_n$ and $e_n$, it seems unlikely that $N(g_0,g')$ could be computed without involving them both -- which it seems is what you've been trying to do.
Here's a plot of "doubling-times", i.e. $N(g_0,2g_0)$ for $1\le g_0\le 100,$ computed using (1) & (2) (it's not monotonic, but is quite close to the hyperbola $g_0\mapsto {30\over g_0} + 70$ shown by the dotted line):
Interestingly, the "doubling-time" is apparently the same ($70$ days) for any starting number of geese greater than $81$ (I checked it out to many thousands).
I don't know how to prove the following claims, though some might be trivial; but all of these are consistent with many thousands of various numerically computed cases using (1) & (2):
Claims: Suppose $g_0=\sum_{i=1}^kg^i_0$, where the $g^i_0$ are positive integers, and that the $k+1$ sequences $(g_n),(g^1_n),...,(g^k_n)$ are all defined by recursions of form identical to (1) with respective initial values $(\text{geese, eggs})=(g_0,0), (g^1_0,0), ..., (g^k_0,0).$ Further, define the pooling advantage as $A_n(g^1_0,...,g^k_0):=g_n-\sum_{i=1}^kg^i_n,$ i.e. the amount by which, on day $n$, the number of geese by pooling the eggs of all the groups exceeds the number of geese without pooling them. Then:
- For all $n\ge 1,$ we have $A_n(g^1_0,...,g^k_0)\ge 0.$ (I.e., as others have mentioned, pooling is never a disadvantage and is often an advantage. It's always an advantage when collecting eggs over sufficiently many days, according to (3) below.)
- $A_{n+1}-A_n\in\{0,\pm 1,\pm 2,\ldots,\pm k\}.$ (I.e., on successive days, the pooling advantage changes by at most $k$ geese.)
- For any desired pooling advantage $a$, there exists a number of days $n_a$ such that $n\gt n_a\implies A_n>a.$ (I.e., for any number of any-sized groups of geese, the pooling advantage eventually exceeds and remains larger than any desired number $a$.
These points are illustrated in the following plot showing the pooling advantage $A_n$, over the time period $1\le n\le 200,$ for the three grouping combinations you mentioned. Here $a*[A]+b*[B]$ denotes $a$ groups of size $A$ pooled with $b$ groups of size $B$:
To help with verification, here are a few traces of the evolution of $(n, g_n, e_n)$, starting with $(0,g_0,0)$ in each case and stopping when the number of geese has at least doubled:
(0, 4, 0) (0, 8, 0) (0, 20, 0) (0, 61, 0) (0, 81, 0)
(1, 4, 4) (1, 8, 8) (1, 20, 20) (1, 61, 61) (1, 81, 81)
(2, 4, 8) (2, 8, 16) (2, 20, 40) (2, 62, 22) (2, 82, 62)
(3, 4, 12) (3, 8, 24) (3, 20, 60) (3, 62, 84) (3, 83, 44)
(4, 4, 16) (4, 8, 32) (4, 20, 80) (4, 63, 46) (4, 84, 27)
(5, 4, 20) (5, 8, 40) (5, 21, 0) (5, 64, 9) (5, 85, 11)
(6, 4, 24) (6, 8, 48) (6, 21, 21) (6, 64, 73) (6, 85, 96)
(7, 4, 28) (7, 8, 56) (7, 21, 42) (7, 65, 37) (7, 86, 81)
(8, 4, 32) (8, 8, 64) (8, 21, 63) (8, 66, 2) (8, 87, 67)
(9, 4, 36) (9, 8, 72) (9, 21, 84) (9, 66, 68) (9, 88, 54)
(10, 4, 40) (10, 8, 80) (10, 22, 5) (10, 67, 34) (10, 89, 42)
(11, 4, 44) (11, 8, 88) (11, 22, 27) (11, 68, 1) (11, 90, 31)
(12, 4, 48) (12, 8, 96) (12, 22, 49) (12, 68, 69) (12, 91, 21)
(13, 4, 52) (13, 9, 4) (13, 22, 71) (13, 69, 37) (13, 92, 12)
(14, 4, 56) (14, 9, 13) (14, 22, 93) (14, 70, 6) (14, 93, 4)
(15, 4, 60) (15, 9, 22) (15, 23, 15) (15, 70, 76) (15, 93, 97)
(16, 4, 64) (16, 9, 31) (16, 23, 38) (16, 71, 46) (16, 94, 90)
(17, 4, 68) (17, 9, 40) (17, 23, 61) (17, 72, 17) (17, 95, 84)
(18, 4, 72) (18, 9, 49) (18, 23, 84) (18, 72, 89) (18, 96, 79)
(19, 4, 76) (19, 9, 58) (19, 24, 7) (19, 73, 61) (19, 97, 75)
(20, 4, 80) (20, 9, 67) (20, 24, 31) (20, 74, 34) (20, 98, 72)
(21, 4, 84) (21, 9, 76) (21, 24, 55) (21, 75, 8) (21, 99, 70)
(22, 4, 88) (22, 9, 85) (22, 24, 79) (22, 75, 83) (22, 100, 69)
(23, 4, 92) (23, 9, 94) (23, 25, 3) (23, 76, 58) (23, 101, 69)
(24, 4, 96) (24, 10, 3) (24, 25, 28) (24, 77, 34) (24, 102, 70)
(25, 5, 0) (25, 10, 13) (25, 25, 53) (25, 78, 11) (25, 103, 72)
(26, 5, 5) (26, 10, 23) (26, 25, 78) (26, 78, 89) (26, 104, 75)
(27, 5, 10) (27, 10, 33) (27, 26, 3) (27, 79, 67) (27, 105, 79)
(28, 5, 15) (28, 10, 43) (28, 26, 29) (28, 80, 46) (28, 106, 84)
(29, 5, 20) (29, 10, 53) (29, 26, 55) (29, 81, 26) (29, 107, 90)
(30, 5, 25) (30, 10, 63) (30, 26, 81) (30, 82, 7) (30, 108, 97)
(31, 5, 30) (31, 10, 73) (31, 27, 7) (31, 82, 89) (31, 110, 5)
(32, 5, 35) (32, 10, 83) (32, 27, 34) (32, 83, 71) (32, 111, 15)
(33, 5, 40) (33, 10, 93) (33, 27, 61) (33, 84, 54) (33, 112, 26)
(34, 5, 45) (34, 11, 3) (34, 27, 88) (34, 85, 38) (34, 113, 38)
(35, 5, 50) (35, 11, 14) (35, 28, 15) (35, 86, 23) (35, 114, 51)
(36, 5, 55) (36, 11, 25) (36, 28, 43) (36, 87, 9) (36, 115, 65)
(37, 5, 60) (37, 11, 36) (37, 28, 71) (37, 87, 96) (37, 116, 80)
(38, 5, 65) (38, 11, 47) (38, 28, 99) (38, 88, 83) (38, 117, 96)
(39, 5, 70) (39, 11, 58) (39, 29, 27) (39, 89, 71) (39, 119, 13)
(40, 5, 75) (40, 11, 69) (40, 29, 56) (40, 90, 60) (40, 120, 32)
(41, 5, 80) (41, 11, 80) (41, 29, 85) (41, 91, 50) (41, 121, 52)
(42, 5, 85) (42, 11, 91) (42, 30, 14) (42, 92, 41) (42, 122, 73)
(43, 5, 90) (43, 12, 2) (43, 30, 44) (43, 93, 33) (43, 123, 95)
(44, 5, 95) (44, 12, 14) (44, 30, 74) (44, 94, 26) (44, 125, 18)
(45, 6, 0) (45, 12, 26) (45, 31, 4) (45, 95, 20) (45, 126, 43)
(46, 6, 6) (46, 12, 38) (46, 31, 35) (46, 96, 15) (46, 127, 69)
(47, 6, 12) (47, 12, 50) (47, 31, 66) (47, 97, 11) (47, 128, 96)
(48, 6, 18) (48, 12, 62) (48, 31, 97) (48, 98, 8) (48, 130, 24)
(49, 6, 24) (49, 12, 74) (49, 32, 28) (49, 99, 6) (49, 131, 54)
(50, 6, 30) (50, 12, 86) (50, 32, 60) (50, 100, 5) (50, 132, 85)
(51, 6, 36) (51, 12, 98) (51, 32, 92) (51, 101, 5) (51, 134, 17)
(52, 6, 42) (52, 13, 10) (52, 33, 24) (52, 102, 6) (52, 135, 51)
(53, 6, 48) (53, 13, 23) (53, 33, 57) (53, 103, 8) (53, 136, 86)
(54, 6, 54) (54, 13, 36) (54, 33, 90) (54, 104, 11) (54, 138, 22)
(55, 6, 60) (55, 13, 49) (55, 34, 23) (55, 105, 15) (55, 139, 60)
(56, 6, 66) (56, 13, 62) (56, 34, 57) (56, 106, 20) (56, 140, 99)
(57, 6, 72) (57, 13, 75) (57, 34, 91) (57, 107, 26) (57, 142, 39)
(58, 6, 78) (58, 13, 88) (58, 35, 25) (58, 108, 33) (58, 143, 81)
(59, 6, 84) (59, 14, 1) (59, 35, 60) (59, 109, 41) (59, 145, 24)
(60, 6, 90) (60, 14, 15) (60, 35, 95) (60, 110, 50) (60, 146, 69)
(61, 6, 96) (61, 14, 29) (61, 36, 30) (61, 111, 60) (61, 148, 15)
(62, 7, 2) (62, 14, 43) (62, 36, 66) (62, 112, 71) (62, 149, 63)
(63, 7, 9) (63, 14, 57) (63, 37, 2) (63, 113, 83) (63, 151, 12)
(64, 7, 16) (64, 14, 71) (64, 37, 39) (64, 114, 96) (64, 152, 63)
(65, 7, 23) (65, 14, 85) (65, 37, 76) (65, 116, 10) (65, 154, 15)
(66, 7, 30) (66, 14, 99) (66, 38, 13) (66, 117, 26) (66, 155, 69)
(67, 7, 37) (67, 15, 13) (67, 38, 51) (67, 118, 43) (67, 157, 24)
(68, 7, 44) (68, 15, 28) (68, 38, 89) (68, 119, 61) (68, 158, 81)
(69, 7, 51) (69, 15, 43) (69, 39, 27) (69, 120, 80) (69, 160, 39)
(70, 7, 58) (70, 15, 58) (70, 39, 66) (70, 122, 0) (70, 161, 99)
(71, 7, 65) (71, 15, 73) (71, 40, 5) (71, 163, 60)
(72, 7, 72) (72, 15, 88)
(73, 7, 79) (73, 16, 3)
(74, 7, 86)
(75, 7, 93)
(76, 8, 0)