Garry's Mod is the host. Sonic Robo Blast 2 is the companion - content is read from your installed SRB2 copy at runtime (rings, springs, item monitors, badniks, music). No SRB2 assets are copied.
Features
Play as Sonic, Tails, or Knuckles
SRB2-style momentum movement toggle
Spin Dash / Thok abilities
Spawn SRB2 badniks from the spawnmenu
Sandbox-friendly with GMod's physgun/toolgun
Requirements
Garry's Mod (host)
Sonic Robo Blast 2 installed (companion, read from install folder)
Content is read directly from your installed games; nothing is redistributed.