The highest AVCHD recording data rate for the GW55 is 28 Mbps - notice the small b, Mega bits per second. What you have written as 12 mbps recording rate you give for class 10 microSD cards is not correct. It should be 12 MBps or 12 MB/s, which refers to Mega Bytes per second, and since there are 8 bits per Byte, that means the card can record at 96 Mbps, easily handling a data stream of 28 Mbps.
I have the GW77, and I'm not sure what the difference is between the GW77 (sold in the US and maybe elsewhere), and the GW55, which is not sold in the US as far as I know. I discovered that microSDXC cards work in it, which is nice, as microSDHC cards top out at 32 GB, while microSDXC card go all the way to 2 TB (but none yet exist at that capacity). Over the holidays BHPhoto had Sandisk 64 GB microSDXC cards for $50. Currently they are $60.
Good luck with your GW55. If it is like the GW77, it should be a great travel camcorder.
Also, when you transfer files from the camcorder to your computer, the camcorder only has a USB 2.0 connection, even if your computer has USB 3.0, so the transfers are OK, but not really fast. If you have an SD card reader built into your computer, it probably is worthwhile to remove the microSD card from the camcorder and use the SD card reader to input data to the computer. For example, in one test I did transferring 2.85 GB of data, it took 4 min 11 s via USB (even though my computer has USB 3) and 1 min via the SD card reader built into my laptop.