We're glad you've read this far and look forward to making this project a success!
For gateway contributions, which are single-file, you should create a /lib/<platform>.dart file to implement the gateway.
Note: do not rely on more third-party packages unless there are special needs. For signatures, if there is no officially maintained signature SDK (depending on it is allowed here), then do not rely on too small third-party implementation packages. You should implement the signature algorithm yourself in your gateway code.
If you find a bug, please report it in the issue tracker.