Hi @rewardy
As a matter of interest, what is this line you want to add? And is it android/build.gradle or android/app/build.gradle you need to modify?
The best thing to do would be to write a config plugin to do your build.gradle change and build with EAS Build. Depending on what change you want to make you might be able to continue using Expo Go. But you might need to build a custom dev client, which is basically like a customised Expo Go with your native dependencies/changes to build.gradle etc. built in.
Here’s an example: Managed EAS build - How to edit build.gradle via mods. - #2 by wodin