If it's a split receptacle, then you have a couple options.

 - Installing a double pole GFCI breaker to protect the entire circuit.
 - Using multiple GFCI receptacles in locations upstream from this device (if applicable).

# Double pole GFCI breaker
This ones fairly easy. Run out to the store, purchase a properly sized double pole GFCI breaker, and replace the old breaker with it.

# GFCI receptacles
If there are outlets upstream from this device, you could install a GFCI receptacle on each part of the circuit. However, the wiring is [a bit complicated](https://diy.stackexchange.com/a/9332/33), and would require you to pull an additional grounded (neutral) conductor throughout the circuit.