Go for the full heated jacket rather than just the vest. Your arms stay out in the wind on the GT, so the extra heat there will help. The only advantages to a vest over a jacket is that it packs slightly smaller, and uses slightly less power. Neither of these is a real problem on the GT.
I much prefer the Warm & Safe
jackets myself. They're hot enough to add all the heat you'll need well down below freezing. And they're better fitted, using stretch panels to comfortably hug your torso (thus ensuring efficient heat transfer). Gerbings makes some nice stuff too, but my $$ went to Warm & Safe.
But whatever jacket brand you buy, you need to get a Heat-Troller
for it. An on-off switch isn't good enough, and the Heat-Troller is the most efficient, most robust heated jacket controller out there. Everyone now uses coax connectors, so the Heat-Troller can be used with pretty much any jacket.
And go ahead and wire up a separate plug for it directly to the battery. Whether you use the coax connecter harness provided, or add a second Powerlet (BMW-style outlet), a direct battery connection will give you maximum heat output when you need it, regardless what the CanBus thinks. You don't even need a relay, as even if the heat-Troller is left full on with the jacket unplugged, it only draws just enough power to run the LED lights so it won't run your battery down even overnight.
Trust me, I've been there, done that with the heated gear and the above suggestions work great for me no matter how cold it gets. Now precipitation/ice is a different issue . . .
'13 Dark Graphite Metallic K16GTLD, 24K miles and counting...
'09 Magnesium Beige Metallic K13GT, 60K miles miles and counting...
'02 Mauve Metallic K12LTC, 106K miles and sold
BMWLT#145, IBA# 366, MOA# 111996, SCMA# 24032
All lower 48 states plus Alaska on the K13GT in two weeks . . .
Some people see the gas tank as half empty. Some see it as half full. All I care is that I know where the next tankful is coming from...