Does it look like this?
Yep - the one on the left in that pic. Without that gasket, the supplied l'il black screws will puncture the core. All rads are shipped from here with them, whether trade or retail sale. They're essential as they also provide the airtight seal between fans to prevent loss-air.

About a centimeter or so?
8mm maximum... 9mm will hit tube. 9+ will puncture the tube.

Screw length = [depth of fan] + 10mm generally (8mm for rad core clearance, 2mm for case/grill thickness)

If NOT going thru case/grill as well, then [depth of fan] + 8mm.