Gumo's true intentions are actually very simple. They're nothing like what he told Lu Sheng—drawing attention from aliens or creating opportunities for Liu Yao. While those reasons are somewhat true, they are not the main reasons.
Gumo's primary reason is to let Lu Sheng show off a bit, to make a grand display, and conveniently, Lu Sheng's Stellar Technique can also be refined.
After all, such a good seedling caught by him—if he doesn't show it off now, is he supposed to wait twenty years and show off on the frontlines himself?
Additionally, letting Lu Sheng display his talent properly, making the old geezers from other factions jealous, letting them realize…
Life Factions! Gumo! Can also train good students!
Such a great opportunity is here, if he doesn't take it… wouldn't it be a waste to have admitted such a good seedling like Lu Sheng into the Life Factions?
Besides…
He's about to leave soon. If he doesn't show off now, should he leave it for Liu Yao?