Both Sungkyunkwan University and SNU (Seoul National University) Yongon Campus (서울대학교 연건동 캠퍼스) are located near Daehakro and subway station Hyewha. Both stations are on Line Number 4 (Blue Line). There is no need to transfer. Also, buses are plenty.
Subway:
Southbound to Hyehwa Station (Station No. 420) from Gireum Station (Station No. 417) - Total travel time expected: 5-6 minutes.
Northbound - just the reverse of southbound
Bus:
Westbound to Sungkyunkwan University bus stop (Bus Stop Number 1003) from Gireum New Town bus stop (Bus Stop Number 8003): Bus number 106 and 109 - 4 bus stops in between and each stop takes about 1-2 minutes – Total travel time expected 5-7 minutes.
Northbound from Hyehwa Maronie Park bus stop (Bus Stop Number 1220) to Gireum New Town bus stop (Bus Stop Number 8004): Bus number 106 and 109 - 4 bus stops in between and each stop takes about 1-2 minutes – Total travel time expected 5-7 minutes.