TG, Tg, tG, tg
Gametes only have one allele per trait because of meiosis. Meiosis is a two-stage process. Before that stage, the cell is diploid, with two versions of each allele. Afterward, only one version of each chromosome gets sent to each daughter cell. Those daughter cells become gametes, and they each have only one version of each allele.
Explanation:
The parents would be TtGgxTtGg because they are heterozygous.
Think of FOIL like this... (Tt)(Gg) and multiply outer and inner
TG
Tg
tG
tg