Answer:
The correct answer would be - GG and gg.
Step-by-step explanation:
As it is given that all the 1000 offspring having the same allele Gg which is only possible in one condition that is cross between GG and gg because it is a heterozygous condition in the offspring. So there must be both allele present and it can not be heterozygous parents as there would be a different combinations of offspring in that case.
If the parents either GG only all the offspring would be GG the same with gg than all the offspring will be gg, thus the correct answer would be GG and gg.