I have an outdoor sub panel that is a 100 A main lug. It is fed from the main panel by a regular 100A 2 pole breaker. I have several things that will be fed off the sub panel for a swimming pool that will need to be GFCI protected for obvious reasons. Can I just switch out the 100 A breaker for a 100A GFCI breaker at the main panel so that all of the circuits that would run off of the regular breakers in the sub panel will be GFCI protected or is this too much discrepancy for a GFCI to trip when it should?
Yeah, if you can find one.
I've never noticed any GFCI breakers on the market that large, but other than that, your concept is sound.
The only issue with putting a lot of stuff on a single GFCI breaker is sometimes, you can have enough devices each with a tiny amount of leakage to add up to enough to trip the GFCI. However that probably won't be an issue here.
I'll add one thing. Now that the common panel is 200-225A (with some 320/400s), when people add subpanels, they often go 100A "because why not? It's only a few dollars more." I applaud forward thinking, but this may help us with the GFCI problem. I have trouble imagining an actual need for 100A at a pool panel. If 60A service will suffice, you can leave the 100A wire and the 100A subpanel just as it is, and change the breaker in the main panel to 60A. (you'll need to pigtail the feeder as the wires won't fit on a 60A breaker). Why would you do this? 60A GFCI breakers are readily available. Bada bing!