Alright so -
For starters, I think the goggle gasket is the little velcro thing mentioned above. I'm not sure how its supposed to work, but I just replaced the front pad of the helmet with that so I could wear goggles and a hat underneath more easily. It's very steezy and I'm sure you won't have a gaper gap problem - just make sure your gogs overlap the hat you have on underneath
Whatever you do, don't wear the goggles on the outside. Personally I think that looks very stupid.
And as for sizing, just look at the sizing charts for both companies and measure your head. If they run small it'll show in the chart.
Apparently, Bern Large is 57-59, and RED XL is 61-63. A RED Large runs 59 - 61. The numbers kind of overlap, so you'll want to measure your head and find out.
http://www.redprotection.com/Default.aspx
http://www.bernunlimited.com/2008/products.html
For the record, I have a Trace and I love it. Peep my reviews if you want to.