From 1.19, PPSSPP Gold for iOS is an in-app purchase in the regular PPSSPP app, so just:
Download PPSSPP on the App Store
Then start it up and click the "Buy PPSSPP Gold" button in the app!
NOTE: If you own a previous version of PPSSPP Gold, there's unfortunately no way to transfer that purchase to the new app. I'm working on trying to find a way. However, you can still use the old app if you have it installed, and it will continue to work, but I recommend switching to the latest PPSSPP, which will be fully featured even if you don't purchase Gold in it. Thanks for your support!
For other platforms, see here.