Both the code and documentation for Gasoline are hosted on github. To get started, here are a few important links.
How to fetch the code for gasoline and compile it to run on your system.
How to run a simulation using gasoline.
What to do if the code crashes.
Sometimes the documentation isn't enough to solve your problem. If you are still running into trouble, feel free to send an email to the gasoline users mailing list. If you are interested in participating more closely, the N-Body Shop Slack channel is where most of the core developers hang out.