I don't know about the JL but I have the JTR and I really love it, It will seat 4 decently. Some complain about the front seats not going back far enough but for me it's fine I'm about 6' foot & have no issues with it. I believe that the Gladiator is basically a Wrangler with a bed so I'm not...
On my Gladiator it was under the carpet, there are some cut outs there that you have to finish removing I used some scissors to finish removing the carpet cut outs. then you will see a rubber piece that needs to be removed. then install the matts, there's a place for the rubber matt drain piece...
I have a gladiator and I was surprised that it didn't come with Red lettering Rubicon Floor Matts, I ended up painting the lettering Red with a acrylic paint pen. The cost of the set was less then $10 bucks, not sure how well it will hold up but worth it to try. I can always purchase new ones...