There's only one officially authorized multiplayer modding platform for GTA, and it's not Rage:MP.
Gaming Community by Max Level on MSN

Take-Two issues cease-and-desist to GTA V modding platform

Take-Two Interactive issued a cease-and-desist to another modding platform. As Rockstar Games consolidates its multiplayer ...
The release date of Take-Two Interactive's flagship game is good news for video-game fans—but it's not helping the stock ...