I just bought a pack of Zebra Sarasa gel retractable pens in assorted colours. According to Staples, this pen "Features a soft rubber grip and smooth-flowing gel ink for effortless writing. Gel ink is water based, acid free and archival quality. 0.7mm medium point. Refillable. Assorted 4 pack consists of port, hunter green, cobalt and indigo."

I have to say, these pens write like butter - it's sooooooooo smooth. It makes writing a pleasure (which was really the point of buying them) and I'm so pleased with them. ...

I just did a test with my Moleskine, and yeah, you can still see through from the other side, but I think that's because you get such a nice rich colour from the pen. The only pen I have that doesn't show through (hardly, anyway) is a Zebra F-301, but that's basically just a ballpoint, not a gel pen.

Anyway, if you like smoooooooth pens, give the Sarasa's a shot - they're not expensive. I normally prefer a super fine pen, but the 0.7 of this one doesn't bother me because it's so nice.