I'm a developer too, I can confirm Git is mandatory when talking about development, there is not a single dev company in the entire world that doesn't use a source control system, and 85 % of them use Git for that. Maybe it's not mandatory for a jam, but everyone should use one for bigger projects. Here is a good video about using Git with GMS:
Another useful tool for a jam would be a "to-do list" tool with more options than just a to-do list, like Trello or Beesbusy. You can log your features to do and things like that, very useful and cool tools. Devs love cool tools.