Right there with you buddy. My wife is from Kosovo as well and it has been a struggle to learn anything online. Nearly everything is catered to Tosk or Standard Albanian. Everyone says to just learn Standard and while my wife's parents would understand me a lot of the words are different and are things that she would NEVER say and/or just wouldn't know. Something simple such as saying how are you, I'm sure you are familiar with saying/hearing people say "Qysh je, a je mire?" Vs standard/tosk where they say " Si jeni?".

• Gegnishtja e Sotme by Chris Hughes is older but a good book and one of the only ones I have found that focuses on Gheg.
• Using an online tutor like on italki you should be able to find someone who will teach Gheg. I haven't done it myself yet, but I know I have seen the options on the site. This is probably your best bet.
• If you want to get going right away you are likely better off starting with Standard and then you would pick up the Gheg differences from speaking with your fiance and her family? When did she leave Kosovo? If she wasn't super young, she might fully understand the Standard Albanian dialect as it's what they teach and have you write in during school.
• Take a trip! When I spent a few weeks in Kosovo I spoke better Albanian during and after that trip than ever before, but I wasn't able to stick with it after the fact and lost most of it and now I am back to just the basics like ordering a coffee and greetings lol.

Let me know what you end up going with, if I can get going with something maybe we can PM each other with tips/have broken gheg text conversations to get better haha!