The female white tabby cat was spotted bleeding heavily with the device attached to her front leg near a barn in Land Gate in Rochdale, on Sunday evening.
It is believed she may have had the trap on her for at least a day as she was spotted with it on Saturday but ran off and could not be found.
RSPCA animal rescuer Steve Wickham was sent to the scene and managed to comfort the cat, now named Xena after the warrior princess, until he could get her safely to the Greater Manchester Animal Hospital (GMAH) for emergency treatment.
Sadly a vet decided her front leg had to be amputated due to the horrific injuries caused by the trap but Xena is now doing well in RSPCA care and once she has recuperated from her ordeal she will be found a new home.
Gin traps are mechanical devices designed to catch an animal by its leg, using spring-operated jaws with teeth or serrated edge.
The device used in this incident is illegal to set and use, although not illegal for somebody to own or sell.
The RSPCA is urging anyone who knows who set the gin trap to contact their appeals line.
The charity is also reminding the public about the laws surrounding gin traps, highlighting how these devices can cause "immense suffering" to wildlife and pets.
Steve said: "Sadly, by setting this barbaric and illegal gin trap, someone's actions have caused this poor cat to lose her front leg.
“But had Xena not been spotted she could have suffered a terrible lingering death, which is just awful.
“She was in such a bad way with her leg hanging on by the skin so the vets were left with no choice but to amputate.
“However she is doing well and will be rehabilitated at one of our branches and will then be found a new home.
“We still regularly deal with incidents involving gin traps and they’re completely indiscriminate in what they catch with victims including wildlife and pets. They are horrific and simply should not be used.”
Xena’s plight was also helped by the RAC after Steve’s van broke down on the way to GMAH.
Steve added: “I would also like to thank the RAC for their help because while I was transporting Xena to hospital my van broke down but they assisted me within half an hour.
“The RAC worker, James Patterson, took me and the cat to the hospital as an emergency then we went back to get my van back up and running.
“They really did go above and beyond and I was grateful for their help as all my other colleagues were tied up on other urgent jobs and were further away."
The use of gin traps has been outlawed in the UK since 1958, but some are still being illegally used to catch animals such as rabbits and foxes.
The sale or possession of such traps is not illegal, but the RSPCA wants to make people aware that they can face prosecution by setting a gin trap.
Anyone found guilty of causing unnecessary suffering to an animal faces a maximum £20,000 fine and/or six months in prison.
The RSPCA is opposed to the manufacture, sale and use of all snares and any traps which cause suffering. Trapped animals will struggle when caught and may succeed in pulling the trap from its anchor only to die later from their injuries.
Donating just £25 could help keep our Animal Rescue Teams on the road. This winter, the RSPCA expects to rescue thousands of animals from neglect, cruelty and suffering. To help our rescue teams reach the thousands of animals who desperately need us, visit www.rspca.org.uk/xmas and Join the Christmas Rescue #JoinTheRescue