Feel free to PM me if you need any troubleshooting information, as I have access to the Subaru Technical Information System
Gas gauge is most likely one of the 2 sending units, I am pretty sure you can get to them by either removing the rear seat bottom, or removing the trunk carpet (I'm not positive since I've never had to get to mine, but I had to remove and clean the sending units in my 90 Legacy, which I got to by taking the carpet out of the cargo area)
You will see a cover secured by three bolts, one on each side under the seat or carpet, one of the two is larger than the other and also contains the fuel pump. To get to the sending units, remove the cover, it is secured with sticky goo so some gentle prying will be necessary to get it off, and once that is off you will see the cover that is secured to the fuel tank itself, remove the bolts and carefully lift the sending unit out. Once it is out you will see the resistive element and wiper, this is what you need to clean with carburetor cleaner, to remove any gunk that may have built up. You should be able to connect an ohm meter across the sending unit and slowly move the float, and you should see the resistance smoothly change, if it jumps around you will need a new sending unit. They run about $45 each from the Subaru dealer.