FWIW, here's my experience:
Geza - nice cover, won't scratch your bike (although I don't have experience with the Calsi screen), fits well, bit pricey. I had the elite series material and for indoor storage it was maybe just a bit of overkill. Not totally waterproof, but close. I used mine to cover both an RT and the GT while carrying it on the back of our motorhome. Never had an issue with quality or workmanship in 2 years constant use.
Sknz - nice well made cover, very similar to the Geza but for less $$$. A bit heavier material than the same size Geza. Has additional webbing/buckles on the bottom that Geza doesn't to secure the cover. Not waterproof, but very water-resistant. Also used this to cover GT while transporting bike on MH with no issues at all. The website doesn't list the 1200/1300 GT but I can assure you the K1600GT cover fits just fine (without saddle bags on the bike).
Sknz found here: http://goo.gl/DlesJ
Pilot Automotive - found on Amazon. Best $17.99 ever spent on a bike cover. With free shipping for Amazon Prime. XL fits the GT perfectly, would even cover with saddlebags, but it might be a bit tight. Not perfectly waterproof as the seams leak a bit, but overall a very nice cover for the $$. I use this when the bike is staying outside while traveling and when the bike is stored indoors. So far it's lasted over 6 months with no issues. Looks like it's still brand new. Available in colors, but per other reviewers, the silver cover seems to be the best for outdoor use.
Found here on Amazon: http://goo.gl/yCYVv