Fuel lines release clamp

GM has used a few different line connections over the years. Early ones used flare nut fittings that took a pair of wrenches. Then came the various quick connect fittings. The common ones are the metal push connect and the plastic squeeze design. The metal ones use a tool that goes around the line and releases the internal locking tabs. The plastic ones use squeeze tabs to unlock. Both types have a second latch or safety clip to keep them from coming off. The metal ones use a U shaped metal retainer while the plastic ones get an internal piece of plastic that forces the squeeze tabs out into the fitting. You need to remove those before you can release either one. The plastic ones can be worse because the safety looks like a part of the fitting. There were a few other used as well. If you can take a picture of the one you have and post it we can give you better advice on removal.