The front end on those G&G Raiders are a weird hybrid setup depending on specific model and year of manufacture...something between a free float and a 2 piece design that the front sight helps hold together. That, and there are about 4 different designs that G&G has made over the years with those front ends, each being different. First, make sure that the only thing holding that rail on the gun is the upper receiver. If it is not a 100% true free float rail, then no, what you want to do isn't possible unless you replace outer barrel AND the rail system. If it is a true free float system and thus only attached via the upper receiver, then all you need is a shorter outer barrel.
For instance, look at the rail systems in your two above links. The second link is a true free float rail as you can see the retaining ring that holds it (between the rail and the upper) in place. Your raider doesn't have this, but it could be the model where the rail system is literally just one entire piece that as a whole screws to the front of the upper. Verify what rail system you have first. If for some reason the rail system uses a method of attachment to the outer barrel up by the front sight block, you are out of luck.
Any "airsoft" suppressor will work for what you want to do. They are nothing more than barrel extensions; the only factors are length and diameter. Make sure what you get is slender enough to fit inside the rail and that it meets or exceeds your inner barrel length once installed.