Gills death wasnt really his fault, predators are not cows, they have binocular vision so can only see 180 degrees, if an alien is hiding in the darkness behind him how is he to know? plus the fact that he was pre-occupied in killing lex, i think it was scars or celtics fault, they ain't go each others back i think one of the other predators were facing in the aliens direction and did nothing.
I wouldn't be surprised if gill was actually the most skilled predator out of the three.