The "New GTS 300..." is a bit outdated (last post jan 2013) so my new figures might be useful for new GTS owners in 2014, for what it's worth.
When it was new, my 2014 GTS 300 (similar to 2013 model, not the 2015 ABS ASR model) used 3.2 liter/100 Km or 73 MPG (US). After some 500 miles its consumption stabilized at a 3.0 - 3.1 l/100Km, that's 78-76 MPG. On longer fast rides or with a buddy this might vary up to 3.2 (73 MPG). I throw in normal gasoline (95 octane).
From what I read about the GTS 300 at fuelly.com, I reside in the most economic league. Neverhteless, I consider myself as a coutious but swift driver. I'm not the WOT type, but far from the grandpa style. I like to kick its tail and leave cars and shifter-bikes in my mirror when the lights turn green
When breaking in the new engine I rode the GTS like I'm used to ride it, exept that I didn't WOT it (well, once in a while to test it). I rode mostly alone but was happy to take any buddy on multiple occasions. Most of my traffic is city and secondary roads, say 30 to 65 MPH and up to 75 MPH. I never drive highways since you need WOT and still, here in Belgium, there are too many crank-heads driving 90 to 100 MPH (instead of legal 75 MPH) passing you with a whoesssh. Now you have an idea of my driving style.
I'm pretty happy with that mileage per gallon, certainly for a swift Vespa like the 300. I honestly don't understand how some 300 drivers only get 50-55 MPG.
I presume fuelly's average 61 MPG includes the constant-WOT drivers and older models.
My brother-in-law has a 250 which runs 4L/100km, some 58 MPG (US) so I expected - at least - something around that mileage, maybe a bit more. I was surprised to see my lhigh mileage confirmed after each fill-up. Apparently, the higher torque and lower RPM's than the 250 at the same speed makes the difference for my 300.
Any figures on rencent GTS 300's are most welcome for comparisson.