Did you write your own gravity component or are you using a plugin? Also - did you figure out your mannequin animation issue? I installed the Mixamo characters and found a tutorial, and was able to sort of determine how animation blueprints work.
First place to check is here - set it to the SK_Mannequin animation BP (from your mesh - inside your player pawn BP). If that doesn’t work, you need to troubleshoot the BP which is harder - or if it’s part of your custom gravity, that’s beyond me. I’ve been using the gravity from this: