The paint stripper from Home Depot. It comes in an orange can and I think the name is Klean Strip. For your application I'd get the aerosol can rather than the brush on. Don't get it on your skin.
Buy a few pairs of long, chemical resistant nitrile gloves, a 5 gallon bucket, a stiff brush (like a toilet brush or something), and 2 cans of Kleen Strip.
Disassemble the gun, put the pieces in the bucket, coat everything for 20min, scrub with the brush and hose off. Repeat as needed.
Seriously, I normally ignore PPE warnings on products, and I just put a bunch of insulation in last weekend in a t-shirt and no gloves, but be very careful and wear PPE with Kleen Strip.
It will absolutely demolish normal purple nitrile exam gloves. You need the elbow length ones that are like 3-5mil thick.
I'm thinking of trying the graffiti off stuff from depot, seems to be a good first option, if that doesn't work maybe I'll go for the hardcore chemies...