The 2008 "National Electrical Code," which most municipalities use, requires GFCI outlets in all bathrooms, kitchens, garages and in any outdoor receptacle -- anywhere water might come in contact with the electrical wiring. Even if you have no plumbing or water in your basement, the code still requires electrical outlets in basements to have GFCIs. If you live in an area that hasn't adopted the National Electrical Code as its standard, you'll need to consult your local building department to find out if they require GFCI outlets in garages.
Testing the GFCI
On the front of your GFCI plug, between the plug recepticals, you'll see two buttons. One reads "Test" and the other "Reset." To test if your GFCI is working correctly, plug a light into one of the outlets. A small night light works well for this, but you can also use a lamp. Make sure the light is on, then hit the "Test" button. The light should go out, showing the GFCI has tripped. Hit the "Reset" button to reset the GFCI and the light should come back on.
When the GFCI Trips
The GFCI measures current coming into the plug and current going out. When there is a variation between these two, the GFCI trips. This could be due to water leaking onto the wiring, to a short somewhere in the circuit, or even to a faulty GFCI. Unplug any appliances attached to the GFCI and call an electrician to determine the cause of the fault.
If you have an existing garage that was built before the electrical code in your area required GFCI in garages, you don't have to replace the existing outlets. But if you do replace the outlet — for instance, if one goes bad — or if you add any new outlets — you'll need to bring them up to the requirements of the current code. And even if the outlets aren't required, they can protect you from electric shock and are cheap insurance, so you may want to install them anyway.