Having just got this I thought I'd share my experience, I was able to farm up 700km over the course of 6 hours on my hd7, my method was to not be on WiFi first of all, and I also set my screen to never turn off.
I went into the game, picked trading post, selected acquire gems through GPS, then when I got to where you check in, I waited a minute, then hit the button, 7/10 times it would give me between 1km and 3km or so.
it seemed like the longer let it stand before hitting check in, the bigger the amount would be, the most I got in 1 scan was 17km when I left it sitting and went to smoke. However there were times where I let it sit for a good 20mins and get no progress
This was just my experience, hope it helps for you, its the only way I could find to make it consistently go up,it took a butt load of time to do but if nothing else is working give it a try, any feedback is appreciated