The MSDS for Gun Scrubber lists
Hexane
Isopropyl Alcohol
Carbon Dioxide
There's a cheap brand of brake cleaner in Walmart that appears to be mostly alcohols, and is probably most similar to Gunscrubber
Note that most of these can be much more aggressive than Gunscrubber. They work great, just keep them away from plastic, pain, or wood. I have noted that Brake cleaners may contain:
Methanol
Acetone
Methyl Ethyl Ketone
Methyl Chloride
Light Petroleum distillates
Toluene
Perchloroethylene
Xylene
etc. etc.
EDIT: Plastic safe electrical contact cleaners are generally similar, but not really any cheaper.