No GSM service coverage is actually available in South Korea. This means no GSM phones (that is, none of your GSM, GPRS, EDGE phones) would work there.
The 3G network coverage mentioned above actually indicates the availability of the UMTS (WCDMA) network deployed by SK Telecom and KTF, and it is not compatible for any GSM phones to work with.
Even if you, say, have a UMTS (WCDMA) phone that works either in Europe or in the US, due to difference in network configurations between South Korea and other countries the UMTS phone would not get service there as well. This is because Korean UMTS network utilizes different bands for up/down streams and other factors.
So yeah, the only way for you to get cellular phone service in Korea would be to rent a phone and then having ur original carrier forward your call to the temporary number you receive in Korea. (This may cost you lots of $$$)