Thats because trunx is half saiyan. And goten too.

Gohan had a tail when he was a child but he is also a half sain isn't he?

Not exactly!
When gohan was kid goku gave him his taile. So this is not his taile.

I didnt know that is it shown in dbz?

yes, he even use vegeta's taile to transform to a greate ape when they were fighting vegeta at the first time on earth after cutting it from him

I thoght it was Gohans own tail. I thoght it grew up again.does the sains in GT n AF have the ability to transform?

In crt0on they make mistake g0han,trunks,g0tain,are half siyan is that right! S0o why g0han has tail and g0tain and trunks has n0t. Next when g0ko is child his grand paa.... Cut his tail and n0t gr0w back, and when pic0lo cut g0han tail it will grew back h0w is this p0sible when goko is ful siyan and g0han is half???