Gloss varnish is...bad. Don't use it. Ever. Except maybe on liquid resin to get a very wet look to it.
With Space Marines, you really don't need to varnish 'em at all, as long as you primed them. Primed plastic seems to hold its paint pretty well, as opposed to pewter which chips at the slightest provocation. If you've got a librarian or chaplain or any models with metal components that are from the older Space Marine ranges, you might want to get some matte varnish for them. I use Krylon Clear Matte Varnish... it's like $0.69 and there's enough in the can for at least a couple squads. At any rate, it's cheaper than GW's, and it works better.
As a side note, matte varnish seems to work quite well with non-metal metallic effects. Not sure why, but it really seems to make 'em shine. Remind me to post some pictures of my new NMM golden-armoured High Elves.