I don't have any problem running MAMP 5.1 on my Mac with Catalina 10.15, and I have not had to turn off Gatekeeper.
Here is a presentation that I gave at GlobalMoot.
MoodleMoot2019 (Global and Mountain), "How to Install a “Sandbox” Moodle on Your Windows PC/Mac"
Here is a presentation that I gave at GlobalMoot.
MoodleMoot2019 (Global and Mountain), "How to Install a “Sandbox” Moodle on Your Windows PC/Mac"