This is pretty tough and I've been there many times. The standard in GAC when going up against big rosters is to get your 10 points and hope they don't show up.
If you really want to take a shot at the GL's your best bet is using your best geared and modded team and trying to pick off the characters around them (unfortunately you probably won't be able to take out their GL cause it will have higher relics and better mods).
In time if you have better and faster mods you may handle a lower geared GL team, but to outright beat them you'll need stronger legendary characters or Proving Grounds characters