I just finished my first level of GG. My goal was no gaps, of course. I used a 0-27 tubular track steel clamp tool that clamps on the tracks and the lever brings them together tightly (fortunately it also works with GG track because of its smaller rail profile than o gauge).
Unfortunately, like model making, it takes a lot of patience to cut the rails perfectly on curves so they line up perfectly and sometimes you actually tighten the track joint you previously joined but that gap is now transferred to the new piece you just cut perfectly and connected. You can keep marking them and cutting them but you soon realize how time consuming it is, especially when you are laying a lot of track. Eventually, in a few cases, I threw in the towel and said, good enough! I agree that gaps from lift outs must exist and I found with my last layout that track tires do wear out faster with gaps. Not a big deal. And IMO the clickety clack is a nice benefit to non-perfection.
I like the idea of using solder or even liquid solder to fill the gaps after the fact. I may try that later.