Gorman was an inexperienced leader who was way over his head, the marines were too cocky to begin with, and the fact that they were going up against an enemy that they had never encountered before left them at a severe disadvantage.
That being said, once the initial shock of the attack has set in, and we lose half of the squad the other marines step into their roles. With Apone gone, Hicks immediately steps in and takes charge, organising a retreat as best he can. Drake and Vasquez do what they do best, shredding everything in sight whilst providing what seems to be effective covering fire, allowing Hicks and Hudson to escape, until Drake goes down defending his squad. Given the speed and ferocity of the xeno ambush, and the fact that over half of their unit, including their leader are taken out quickly, the survivors adapt very quickly. Hudson is obviously the one who panics and kind of crumbles during this, it makes sense because he is the cockiest of them all, but even he goes down fighting whilst the others escape.
Then there is Gorman, who I feel is often overlooked. He's obviously not cut out for the job, and that becomes apparent as the nest ambush goes down and he is reduced to a gibbering wreck. After he wakes recovers from being knocked out he changes, he steps back and allows Hicks and Ripley, who are both far better at leading people, and he immediately falls into line. If you think back to Hicks, 'Looks like the new Lieutenant's too good to eat with the rest of us grunts,' and then consider Gorman's final act, he goes back to help Vasquez, at the cost of his own life, it goes to show the changes he went through as a character. From the arrogant 'rupert' who didn't want to connect with his team outside of barking orders, to a soldier who was willing to die for his comrades.
Overall, I think the marines did the best that they could, given the overwhelming situation they found themselves in. If they had previous experience with the xenomorph then they could be considered to be incompetent, but they didn't, so they shouldn't.
