I also got a 16 GB microSD card and I didn't have to do anything. In fact I was able to transfer two 1.4 GB movies on over so I can watch them on my G tablet. I would think that reformating them using FAT (on a windows or Linux machine) should do the job.
The OS/Software Services has to scan the card for new media. Have the card inserted and power up the device as it runs a scan on boot. I have not seen it run scans at any other times.
If the card is in and you power the Gtab on, it should scan for new media and allow you to see the pictures, etc.. in the gallery.