PVP IV is an app for iOS and Android intended to help you rank up in Go Battle League. It has features such as customizable Pokemon and team rankings, an IV ranker to help you pick the best IVs for each league, and a highly advanced 1 vs 1 battle simulator. Note that some features are iOS exclusive.
PVP IV’s latest feature is Custom Metas (iOS only), which I used to finish the season Top 5 in the world in GBL! This feature allows you to build customized rankings based on the Pokemon you see in your battles. Learn more from this help article and this Reddit post!
Here are some help articles with more information: