Fun fact about the license under which the Mastodon source code is published (AGPL) is that it requires any modifications to it to be published.
No secret sauce, no proprietary features possible. Which is why commercial companies hate this license.
Want to paywall "pro" features? You can, but anyone can take your code and run it on a different server for free.
Anyway, that's quite an interesting financial choice for Gab, a for-profit entity that sells "pro" features