I don't see the problem with it? Future Gohan has had enough. He's done with the chaos, the brutality, the anarchy. It ends now. He feels he is strong enough to be able to pull it off, but it's highly risky, he has made the decision to go through with it, no matter what. Trunks wants to offer help, but Gohan knows it is the last chance for humanity should Gohan fail. So he does not allow Trunks to go with him, because he is humanity's only hope should he fail. He's not hoping to fail, but he knows it's a possibility. Unfortunately for Gohan, the Androids were much more powerful than he expected and they take him out. However, him not allowing Trunks to go is the exact reason why his timeline is saved. Trunks and Bulma figure out how to save their world and it doesn't happen without Gohan.
What is there not to understand bud?