I don't ride an awesome 1200GS in any of its colourful variants, but...
Any bag becomes waterproof if you put a half decent plastic bag or two in it, trust me. You might well save yourself a decent slug of wedge at the same time. More money on beer and motion lotion, as a consequence.
How to hold it on? Again, it depends on how much you might want to spend or to some degree or another your aesthetic sensibilities. Anything from string, through bungy cords / cargo net, the free (if you ask) straps that hold BMW motorcycles into their transit crates from Munich into your local dealership's showroom, right up to the latest RokStraps and similar. If, as many are, you are conncerned (or even terrified) that someone will steal your bag as you hoon or pause to press wild flowers / chill in the cafe with likeminded souls, you can sling something like a steel mesh cargo net over it and then padlock it in place. Don't lose the key, is my only tip.
Will it / might it rub? Yes, of course it might. Just use something that will help to stop it rubbing. Again, it's up to you what that might be. Start with an old towel, perhaps? Or some of that non-slip matting? Or a length of gaffer tape, maybe? If you want to go the whole hog, there's clear protective tape / film you stick on that'll probably outlast your awesome steed's engine and frame paint.... it may even outlast you, if you are very unlucky.
So there you have it OP. If you want the look of a rugged self-reliant Adventure biker, it's down to the poundshop or army surplus with you. The scuffs and rubs will come free, adding to the bike's provenance and yours, as a true Adventure biker with steed. If if it's Touratech Adventure image you gravitate towards, it's their bulging catalogue you'll need. Either will do. Nobody will really mind, not least as they may never know.
