mapbox-gl-rails | provide Mapbox GL JS and it 's ecosystem | Application Framework library

Gemfile.lock is a file generated from Gemfile. As such, instead of trying to merge the two branches, it's simpler and more accurate to generate a new one from its canonical source. This might result in slightly different versions, but these should cause no trouble; any version restrictions should be defined in your Gemfile.

Normally one does not commit generated files, they can change in trivial ways, but Gemfile.lock is a special case where you do want this to be the same for all builds.

Resolve any conflicts in the Gemfile. Regenerate Gemfile.lock. Add it.

for some reason he copied the code and added it himself

This is a good opportunity to explain to them why this is a bad practice when working with a team. It might be easy for them, but it's causing trouble for you. They might need instructing in how to update their work in progress. Or you might need to extract some changes into their own branch and get that merged.