The punch absolutely rocked him and
if their heads clashed, it was after Rumbles head stopped its forward movement and reversed direction, as he was standing back up.
Lets not forget that wasn't even the end of the fight. Gus was still in it and fought for like 30 seconds and exchanged striking, and had quick footwork movements after.
http://fat.gfycat.com/OblongDeadApis...alaboriosa.mp4
http://giant.gfycat.com/WelllitSillyGardensnake.mp4
http://giant.gfycat.com/NearPlasticAiredale.mp4
edit: i dont know how to gfycat