Thanks!

Yes, it ended up having nothing to do with the "Real Tech Tree" but instead was a weird side effect of including JFD's Chilean Civ/Leaders, even if they're not in your current game as a player or AI controlled Civ. Just including JFD's Chilean pack in my currently selected mod configuration tweaked the game. JFD even admitted, once I an another player brought it to his attention, there was something weird going on with his Chilean pack, but it would appear resolving this is not a high priority for him. As far as I can tell, other than a few of JFD's Civs not being updated to run with The Gathering Storm, those that are recognized by the game as compatible work fine other than this Chilean group (I think it was the Chilean Civ and 2 more alternative leaders).

Hello there--I was reading the comments on infixo's "Real Tech Tree" thread, and observed that you were having an error that I am experiencing--unable to work Iron until Gunpowder. Did you ever discover which mod was causing the issue?

@antananarivo Thanks for the offer, but I'm good.

HAPPY NEW YEAR!