Not the sharpest tool in the box, are we? He's tangible when the fan hits Naruto, yes. But look at the panel next to Tobi slipping through the rock: Naruto is holding the fan in his hand. If it was to change its state along with Tobi, it would've slipped through him. After all, there was still a momentum.
It may also be badly drawn, but it pretty much looks like I just explained it.
Thing is that they never do collide. The moment he's touching Gai with his hand it's over anyway, since he'll suck Gai into his dimension. It's not like Tobi can turn anything intangible that touches any random part of his body. Even if that was the case, Gai is fighting with his nunchucks; they're the only that would turn intangible.
Gai even stated that he's using them so he doesn't make contact with Tobi.