How to determine which gems an app depends on?

I have an app for which script/server crashes on startup, probably
because the platform doesn't have one or more required gems
installed. I'm wondering if there's an easy way to determine which
gems are required or missing. Thanks.

You need to look for require statements that are failing. Start the
server in development mode so you can get more explicit errors in the
logs.

RAILS_ENV=development script/server

tail -f log/development.log