To be honest there isn't a huge selection of Gunblades on the market and with the recent event of
Square-Enix cracking down on those who infringe their copyright there may be a whole lot less to choose from.
So if your looking for an official life size
Final Fantasy 8 Gunblade, I'm sorry but your out of luck. The two I know of on the market are Squall's Gunblade and Seifer's Gunblade both of which are 1/6th scale but beautifully crafted by Square-Enix.
If you're able to find them anywhere else at a different scale then they're probably not legal, Square-Enix only produces the two above.
But what other options do you have, well if you don't mind imitation then your in luck, they're not exact replicas of those found in FF8, but they are "Gun Blades"
We'll start with what I think is the coolest of the non FF8 Gunblades, the X Fantasy Gun Sword (no longer available), this is a life sized Gunblade with steel blade and cast metal handle and comes with a sheath, the handle looks a little naff though. But if your after a life size steel Gunblade I think this is your best bet.
Then we have this piece (no longer available), it looks pretty cool and at 42" it's rather life size, I think it's made entirely of wood, which would make it perfect for cosplay.
Following the above items there really isn't much else, below are a few pretty unique Gunblades, but really nothing like what's in FF8.
Pirate Flintlock Pistol / Dagger Combination
1800s Flintlock Pistol / Dagger Combination
Flintlock Fantasy dagger
And lastly we have what I think is the coolest of the lot, it's not life size so that's why it's last of the list, but I think it's even cooler the FF8 Gunblades, it's got a stainless steel blade with a hardwood handle and the cast metal barrel and trigger look quality as. Awesome looking Gunblade.
Revolver Gun Blade (no longer available)