Some more silliness from Garmin
You can only edit the gas details on Garmin Connect Mobile (this may be temporary if they are waiting on updates to the web interface).
The editing capabilities are short sighted.
When entering the tank size in litres you can only use an integer. I have a number of tanks that I use and they are all Faber. Faber is a widely used brand of tank (at least in Australia). I have 5L, 7L, 10.5L, 12L, 12.2L and 15L tanks. If I use the 10.5 or 12.2L tanks (which I use most often), I can't enter the correct tank size. It doesn't sound like much, but it is required to accurately calculate SAC - not that Garmin have any clue about that.
So, SAC. It also only allows an integer. My SAC ranges from around 0.8 to 1.6 bar/min. An integer is completely meaningless. Didn't that Garmin guy say the developers were divers? Also, it doesn't calculate SAC. You have to enter it manually.