Have we seen the same movie buddy? Gohan has no idea Piccolo got jumped at all.

Piccolo goes to him and says "Train a bit every once in a while" as Gohan is working on his project.

Only after Piccolo inflitrate in the base and wants Gohan on board but doesn't tell him anything, he simply uses Pan as motivation.

Gohan has no idea there is a new threat till they quite literally knock at his door and had Piccolo watch over his daughter.

Also yeah Tien kinda travels around the world without the characters knowing much where he is like 90% of the time. Still, his strenght wasn't really enough. They used the people they can reach quickly: 18, Krillin, Goten and Trunks