As long as it's not the "repositionable" kind that makes a very weak and short lasting bond. You should be using stuff that claims "permanent" or "high tack."
If you are using black Gator make sure the adhesive is the kind with a white center section, otherwise the black Gator call pull down highlights with thin print media. A lot of companies like Artgrafix skin their own sheets, and I have to wonder how good they are about avoiding grit and stuff that shows up big time with thin inkjet papers. If prior to mounting you see a grit bump on the release liner, you can cut an X over the bump and peel back the 4 petals to reach the grit which unfortunately will still be under the adhesive, best to use a #11 Xacto blade and maybe crisp-edged tweezers. Don't do any X'ing until just before mounting.
30 x 40 is on the edge of what I would want to do without a roller laminator, and then only with a pre-applied adhesive. After you do the 1" peel back and tack the print at one end, feed the print onto the adhesive as though it was coming 180 degrees around a roller, pressing down inside the curve to simulate roller pressure. This should be a two person operation, one pulling back the liner as the other rolls out the print. If you have to do it solo it's probably better to completely remove the liner before starting the roll out. It might be worth your while to buy 36 x 48 Gator until you've had some practice, takes away some of the stress by giving you some alignment leeway. I've mounted prints that way, but a lot of them had bubble issues pretty soon afterward. Best to get a laminator. A friend just picked up a 39" model from ebay for $165 shipped, it's pretty decent with 3.25" rollers, as long as you're only mounting prints or skinning Gator with adhesive and don't need to adjust the pinch very often.
3/16 Gator would be fine for 30 x 40, lamination doesn't create the heavy warping you get when gluing Gator. You could also use 3/8 or 1/2" foam core which is a lot cheaper.
I notice Codamount.com has black gator skinned with their white opaque Coda Mount 1 adhesive, for $28 for a 30 x 40. I'd be using it if they weren't on the other side of the country.
You might want to look into adhesive with a double release liner, which has a removable liner on either side of the adhesive. You can cut pieces to size before use, and it's just a whole lot easier to deal with when you don't have a laminator or you're an adhesive virgin. Most of the those don't have an opaque white center section, so you would need a white substrate.
