Contreras asked me to find Isaac in the Gun Runners outpost north of MCarran, but the gate is locked and if i go in i get shot at. Theres only a robot outside selling weapons and i dont know what i have to do exactly.Am I at the right place?

he usually comes around during the day otherwise go look for his house its nearby

His house is east of the gun runners by the overpass. It's the second house. The first one has a very cranky woman in it.

Sometimes he walks near the entrance of the gun runners.

He is usually there during the day, and his house is not far away. Poor Isaac, everyone complains about him - either he's not there when they want to meet him, or he's hanging around when they want to break into the factory. I feel for that guy, he can't win.

alright thanks for the help

if hes not at the desk in front of gun runners hes for some odd reason stuck in his house... if hes not in either of those spots hes probly dead

This, he gets stuck in his house to the east of the gun runners, near the hostetler home, which you might have been in for a quest.
Once you go in his house you might see no one is there... that's because he left as soon as you went in. Go outside and you'll see him running. Run after him, you'll never catch him- he's fast. After all, he's late for work. But don't worry, he's just running to the gun runner's kiosk, when he reaches it he'll lean over the reloading bench there and go to work, and stay there, seemingly forever.