I'm in the process of putting together a sticky for closing IG pools similar to what Watermom did for AGs. I'll post then solicit comments and additions. On the gizmo issue, I don't rely on that solely for expansion, I pack the skimmer with noodle type closed cell foam. Gives a lot more room for expansion. For the skimmer pipe, I blow that dry, inssert gizmo, then fill the pipe with RV antifreeze up to the pump inlet which in my case is higher than the pool water level. If the gizmo leaks the AF will run into the pool. Another step before the AF is to pressure test the line to make sure the gizmo threads don't leak...most people don't do that. I also don't drain below the skimmer because of ground water issues.
Al