What each app is built for
Hevy is built around making logging fast and the history easy to read. Its paid tier removes the free limits on routines, custom exercises and history length.
GYM RPG logs the same sets, reps and weight, then feeds them into a game: each completed set earns XP and a muscle stat point, points go into a skill tree, and the tree sets how hard you hit a boss your party fights together.
Free tier and pricing
Both apps let you log without paying. Hevy's free limits are on how many routines, custom exercises and months of history you keep. GYM RPG's free limits are on saved plans (3) and how many workouts per day earn XP (1); you can still log more.
On price, Hevy is cheaper at every tier. GYM RPG's Pro is $7.99 a month or $49.99 a year on the US App Store, and it includes game-side extras such as daily gems and extra chest slots.
Where Hevy is the better pick
If you do not care about a game, you are paying for features you will not use. Hevy is a mature, focused tracker and costs less.