I need to dialogue because it looks like Gunko was going for Usopp after he blew her up and then Nami stopped it by grabbing the arrow?
That would make Gunko's ability less broken.
It's likely some "determined to get there" type of hax.
When she slid or ran into him she could also go underneath that arrow path without falling off.
Seems like the arrows' determined paths overwrite physics and stuff and there's barely anything you can do about it (besides just nuking Gunko I guess).

