Ok, lets get this out of the way. GSync and FreeSync are monitor-level features. They're not something that you "feed" your monitor. Both types of monitors are "variable refresh rate".
In a nutshell:
FreeSync is (for intents and purposes) = VESA Adaptive Sync.
GSync monitors contain a proprietary chip that receives the frames from the GPU and tells the panel when to refresh. They have this because GSync was released (a few months) before the VESA standard was ratified.
Two ways to achieve the same result.