The class doesn't really matter all that much, read speed and class rating are only vaguely related.
The general rule is that higher class cards have a faster write speed, but a slower read speed. For the Gateway you really only care about read speed, so a lower class would be better in this situation. That means, pretty much anything but a class 10, but kingston class 4's are rather fast. Sandisks are much slower from the benchmarking I've done, though that benchmarking is limited to a couple of cards of each brand.
If you get a class 10 MicroSD labeled UHS, you can be certain the read and write speeds are both high, the Gateway won't take full advantage of that due to not supporting the UHS standard, but it will give you better speeds than other cards.
However, it's overkill unless you can find them for a really great price (as cheap as a lower class), Amazon has great prices on them sometimes.
I had a transcend. It broke within a few months and the data was not recoverable.
Also, it was rather slow, according to some benchmarks I performed.
Suffice to say I'm never getting another Transcend card.
That UHS card is probably overkill, but for the right price, it's a good choice.
