But if Gohan wasn't around to teach him how to fight, Trunks would have died a long time ago & wouldn't have gone back in time
Trunks did good on his own for well over a decade. Gohan's training didn't even get Trunks anywhere in the future in the long run. He got beat when he encountered the androids after Gohan was killed. Which of the two survived their whoopings?. Those genes the inherited resilience. Whose training actually mattered in the long run?
Vegetas
He pushed his boy and unlocked his potential which led to his son saving his future timeline. Meanwhile Kakarots son's power drastically deceased and he started playing dress up, that aint no saiyan