Interesting excerpt from Steamworks docs:
Which Open Source licenses are compatible with the Steamworks SDK?
In general, permissive licenses that do not put any requirements on you to redistribute your modifications under an open source license work fine. Common permissive and acceptable licenses includes MIT License, BSD 3-clause and 4-clause, Apache 2.0 and WTFPL.
Which Open Source Licenses are problematic for shipping on Steam?
Generally, any license that has a so-called “copyleft” element will be problematic when combining code with the Steamworks SDK. The best-known example is GPL.
But I saw a GPL-licensed application on Steam!
This can happen if the author of the code that is GPL-licensed has given the permission to do so. The author can of course always (a) decide to grant Valve a different license than the author grants everyone else or (b) decide that what the Steamworks SDK does is just a communication with a service that does not invoke the copyleft requirement of the GPL.
Sounds like (b) above could apply to you?
I believe this would require agreement from all contributors, or for them to sign some kind of contributor licence agreement.