There is no way to browse to that level of detail on Fuelly at this time. We're considering ways to allow people to add more engine details, but at this time for subaru legacys we only ask people to distinguish between the H4 or H6 engine and whether it's a sedan or wagon. You can see that listed at the bottom of each vehicle listing on the "driven by..." line.
Oh, and you could always add a custom model called "Legacy GT" if you want to track your mileage in a separate model, with its own page and statistics. Then everyone who comes along after you would see "Legacy GT" as an option when they create their car with Fuelly. That might be the best way to go right now.
It would be nice if people would just write it in the comments about their car. With the sentra, you have the 1.8 and the 2.5, so they all say "L4," and you can't compare your car to others with the same engine.