In character Genthru would not use any nen against someone who doesn’t have it. His overconfidence is his biggest weakness. He’s already in the mindset to fight, and he has no reason to scare Deku like he did others, so he would 100% start with trying to beat him up physically. He’s going up against a kid with no Nen, he’d try to establish himself as superior first before revealing Nen.
Not Really since people without nen have been effected by Nen. Though Genthru might be stupid and try to fight Deku Nenless. If both are in character & the situation is Deku arresting the Bomber I think initially Genthru would be caught off guard but, would likely go for the contact bomb.
Genthru loses here for the same reason he lost to Kira. His overconfidence in his ability to crush people with ease, especially people without Nen, leads to his downfall. He wins if he flexes his nen at all, but he’s so insanely overconfident and such an egoist he wouldn’t do it as a first move.
Isn't there a difference though, Deku is likely to Rush in with Full Cowl or prepare a Amp punch for Genthru at that moment Genthru will notice the enhanced speed, the blue energy, or the red glow on Deku's fist & nen up.
That point was already discussed. Genthru is so overconfident he wouldn’t feel the need to activate his Nen against someone he perceives as both a normal person and weaker than him. He’d go for physical combat 100% of the time in this scenario, and he gets 1-2 shot for doing so.
If he uses Nen at all he wins, but he’s so overconfident he won’t. If he survives the first hit and isn’t so disoriented he can’t do anything like he was in his fight with Gon, he wins. If he doesn’t survive the first hit, or is too disoriented, he loses.