I've had my headspace away from this for a little while, and didn't browse my usual haunts of LEMB website and all my saved documents, this is all just off the top of my head but should be about right.
Okay firstly the 350kg GM-1 tank was pressurised and not interchangeable with the 150kg MW50 tank.
GM-1 was used on the 605A-1 motor, it was tested on a couple of early build G-1 and serially fitted to some G-5 (an option for G-6, possibly used on a couple in Med). It is useless for the 605AS motor since it can't even be used on that engine under 9km altitude and B-17s cruise over Europe at 5.5-6.5km alt. A regular G-5 or G-6 with a 605A-1 motor and GM-1 will have a nice burst of speed at around 7km alt, though which is right where B-17 escorts like to hang around. It's good for them, just not the AS.
Point is, GM-1 is useless on an AS motor.
Second, MW50 engine kits did not enter serial production until February 1944 and so weren't delivered for fitment into aircraft until Mar-Apr and so weren't seen in service until about Apr-May as Anto mentioned.
Next point, very few G/AS were built in the first half of 1944, Me-109 production shot up in the second half of the year and it was all anyone could do just to get enough regular fighters to the numerous, ever changing fronts all over the Reich and occupied territories. A small number of G-5/AS (by small I mean like 20 maybe), and G-6/AS (something like 70-100 sounds about right) were built and delivered for Reich Defence (ie. Berlin defence, mostly as wilde sau). If you actually jumped in a keubelwagen in say May44 and went hunting for G/AS of any description being used actually in service, you'd manage to find like 2 or 3 dozen in the entire universe readily. They didn't have MW50.
A couple of MW50 kits did make their way into some G-6 just ahead of G-14 production but universal convention among Luftwaffe pilots is to call these G-14 anyway. The G-14 is really an industrial designation, they are G-6 and G-8 just with MW50 serially fitted, the significant thing about them is a reorganised industrial schedule to dramatically increase Me-109 production output. They also had a radio nav update because new towers/signals for all weather operations in the Reich were being used from about mid 1944, but you wouldn't need that to operate in a lot of areas (eg. SdOst, Baltic, N.Italy). New build G-14 also got a new instrument panel and an MW50 guage but a great many G-14 are just converted late build G-6 with the conversions done in the field and they didn't get that, just the radio and MW50 kitted motor.
So basically if you found a G-6/AS with MW50, it wouldn't be before Jun44 and everyone, I mean everyone would call it a G-14/AS anyway. The only difference between such a thing and a G-14/AS is if the G-14 is a new build (after July 44), then it will have a different instrument panel and a new serial number will be stamped right over the top of its factory tooled G-6 serials. A lot of listed G-14 didn't have these however, they were just late build G-6 conversions in the field, it is really in 1945 that the G-14/AS became very distinct because it started getting the ASB/C motor, which brought it to K-4 standard like the G-10. The regular G-14 though was a real hodgepodge, they were getting whatever was laying around the factory floor by then, often just a stock non-boosted 605A-1, sometimes a 605DB that belongs in a G-10.
An interesting sidetrack. Hartmann refused a new build G-14 in mid44 because the build quality was so bad. So he confiscated a 1943-build G-6 from the ErgGr training squadron and had it updated to G-14 standard. That is to say, he had an MW50 kitted motor fitted. The wk.n remained listed as a G-6 however, so there has always been a little discrepency about Hartmann's second last mount before his G-10, whether it was a G-14 or a G-6. It was a G-14 but converted from a G-6, I don't think it even had the erla haube.