Do I need UHS-1 card for GX1?
Any class 10 card will work fine. Heck, even class 6 or class 4 cards should be okay, unless you're taking lots of rapid-fire bursts of RAW+JPG. I've got a class 4 16GB card that I've used a few times on my GH2 with no problems, even for the highest quality video (on stock firmware).
The only time I could see a need for the the newest, fastest 95MB/s UHS-1 cards (like Sandisk Extreme Pro) is if you want to experiment with some of the ultra-high bitrate video options (like 176Mbps) that people doing with hacked firmware. But the GX1 has just been released, so I wouldn't expect its firmware to get hacked any time soon, and by the time it does (maybe a year from now), hopefully the cost of such cards will have come down significantly.
Until then, I'd say anything beyond regular class 10 cards is probably overkill.