Stripping paint off galvanized steel gutter

I scraped about a third of the paint off. What tool should I use next?
Bosch Random Orbit Sander?
Angle Grinder?
Wire brush on Hand Drill?
Hot air gun?

A good pressure washer

I would replace the gutter. There are better choices now than galvanized steel, and it would take less time to replace it than prepare to repaint.

Took me years to realize this but if the paint is hard to remove it is sticking to the substrate and should be left alone as a primer. Now if you want a dead level finish you have to remove the good with the bad or add filler.
Bil lD

I've been removing rust and old bottom paint from our steel hulled houseboat for longer than I care to admit, and I have nearly all of Harbor Freight's north wall of sanding and grinding accessories under the boat- The best thing I found for what you want to do is this:
Attachment 414236
It's slightly cone shaped, fine dense wire, and much more aggressive than one might think, and at the same time once you're to the metal it's easy on it. If you have paint sticking good in spots and don't necessarily need or want to fully remove it, this brush will feather the edges nicely.

I used mine just a few hours ago to remove the foamy white residue from the phosphoric acid I've been slathering on the bare metal I've been sanding into. Using light pressure it removes the white powder but doesn't cut thru the acid-etched metal. But it will if I want it to ;)

You can pick up one of their angle grinders AND the wire brush for $15... :)

You run the risk of removing the galvanizing using power tools. If you do that, the rust will come with a vengeance.