Switching trackers shouldn't cost you years of receipts. If you can get a CSV export out of Hevy or Strong, GoGAINZ will read it.
Before you start
You need to be signed in, because your history syncs to your account rather than sitting on one phone. Importing itself is free on every tier.
Get your export
In Hevy, go to Settings > Export & Import Data. In Strong, go to Settings > Export Data. Both give you a CSV file.
Import it
In GoGAINZ, go to Settings > Data Import > Import workout history and choose the file. Before anything is written you'll see what's in it: how many logged sessions and workout templates it found, how many exercises were matched, and how many need review.
Reviewing exercise names
Exercise names GoGAINZ recognizes are matched for you. For the rest you decide one by one: match it to an exercise already in the app, create it as a custom exercise, or skip it. Nothing lands in your logbook until you've been through the list.
What comes across
Your logged sessions, the workout templates behind them, the personal records inside that history, and the RPE you logged in the other app.
What doesn't
Notes, distance measurements, empty sets, and warm-up or cool-down entries. Rows the file can't be read from are counted and reported at the end rather than guessed at.
Running it twice
Importing the same file again updates what's already there instead of duplicating it, so re-running an import is safe. That also means you can import, add more history in your old app, export again, and import the newer file.