The core difference
Strong keeps the experience narrow on purpose: log sets, see charts, use templates. GYM RPG keeps the same logging and connects it to a character, so consistency is rewarded with levels, skill points and boss damage.
Free tier and pricing
Strong's free tier logs unlimited workouts but caps custom routines at three. GYM RPG also caps saved plans at three on free and rewards one workout per day with XP; logging itself is not capped.
Strong PRO is cheaper per month and per year, and it has a $99.99 lifetime option. GYM RPG Pro is $7.99 a month or $49.99 a year with no lifetime tier.
Where Strong is the better pick
For a no-frills logger, a lower subscription and a lifetime purchase, Strong is the stronger value. GYM RPG is worth the extra if the game layer changes whether you train.