You can play campaign if you sign off of live. They are connected incase you decide to do co-op with someone over live.
Sign out, and you can play campaign. All of Live is down right now however, it's not just gears.
edit: the downtime was only supposed to be 20minutes, and there was rumors flying about for alot of reasons why that might be. I think we are at about 1.5 hours though, and one of the support guys mentioned it was a big problem and the dev team was being called in lol. Not that big of a deal, I can't remember a time when live ever went down, and i'm so used to it with ffxi and wow I guess it just doesn't bother me at this point.
edit again: just found out its been down for almost 3.5 hours now lol. One of the tech support guys said ~4hr's though, guess we'll see. I really want to play too