Nah it wasn't a partial Gear 2 on his arm.Don't be ridiculous. You know Luffy can decide which part of his body he turns in gear second, right? For the red hawk he only needed to turn his arm like that, not his body. He wasn't moving at maximal speed
You must be registered for see images
You can see steam coming from his hat, shirt and left leg, and his right leg is all blurred out, which may be an indicator of fast movement.
I realized something though, Luffy may not have been traveling at his Gear 2 speed after all, since the snap of punching his arm at such speed while it's stretched out should cause the momentum I was talking about. It explains how he couldn't close that short distance before Trebol finished talking, and he didn't actually launch his arm until after Shambles, so yeah, I was probably wrong.