I agree with Lob on that one. Let's also not forget that Swiss do their schooling in high german. Well they are supposed to, but these days many don't. I've met some Swiss who complain that they'd rather speak English than high German, but this is from Swiss that have excellent command of English.

So the bottom line is that you will not disadvantage yourself by learning high German, and you will get major brownie points for putting in the effort. There aren't that many places or books where you can learn Swiss German, since almost everybody learns high german (then has to leave the classroom and deal with something completely different on the street).

You'll find that your girlfriend's parents and friends will automatically speak to you in high German if they hear that you are struggling (which you will!!), but more likely they will just switch to English because it will probably be more comfortable for them than listening to what will be broken German Sorry...

Have you spoken to your girlfriend about any of this? I mean she is Swiss, she knows her own family, and surely she will have some advice for you.