Steve Kenson and Crystal Frasier kind of answered this in M&M Monday.
(in response to Troy asking what Grab-Based was after someone asked if it worked as a Reaction)
Crystal Frasier: There's a Flaw for your power where it only goes off
Steve Kenson: Off when you grab someone. It requires a grab in order to use the the power essentially.
Crystal Frasier: ...
The Grapple Gun is, as you discovered, Movement (Swinging) with a technology descriptor, to move around like the Batman: the Animated Series version of Batman.
The line has an 'appropriate' length to allow the character to swing about the terrain. The GM can impose a complication, rendering the gun useless in the current terrain, for a Hero Point. Just like ...