That cig lighter heating up pulls a lot more power that that Garmin. I know it is for a shorter time frame but still the Garnmin is a trickle compared to the lighter. Although I have not used it to power a GPS I use it to charge a cell phone all the time on my 2010 Limited.
I use the stock cigarette lighter port to charge my phone, to power my heated clothes, and I used to use it to power my xm before I got the unit under my fairing installed. No problems so far.
You should be okay...I use mine to power two 500 watt halogen lamps, a coffee maker, griddle, computer, television, satellite and DVD player, all at the same time...It does blow a fuse when I hook up the hair dryer though .
Actually, I leave my mp3 player plugged into it for hours on end when I'm riding and I'm yet to have any issues.
I made a harness to from the battery charger connector behind the side panel, up to my windshield bag. I used it to charge my iPhone and run a pump for an air mattress while camping. I had to up the fuse on the charger circuit to 15 amps though. When I don't need it, I disconnect it and put it away.
If it'll take the current draw of a cig lighter, it'll certainly take the 1-2 amps of a GPS... I've run mine their for days at a time, main reason is probably cause any power plug for GPS or other device will hit the tank if you turn full left ****... Just be cautious of this.