Looks like silicone mold putty. Several of the major silicon & resin companies make versions of it. I have some of the stuff made by Alumilite (theirs is yellow rather than blue). It's is very nifty, though you have to be very careful to avoid folds or creases when squishing down onto the pattern, which can occur very easily because it's chewing-gum like in consistency (very similar to GS), but slippery/oily instead of tacky.
If you live in the US, the Michael's craft store chain carries the Alumilite version, which is called "Amazing Mold Putty". Make sure to wait for one of the 40-50% coupons though, as it is rather pricy.
Incidentally, you want to treat this the same way as epoxy as far as handling to avoid cross contamination between the A and B parts. I cringed a bit watching the video where he scoops directly from both containers with the same fingers.