Yes Seoul Station AREX (Airport Express train line) are for everyone. It's just the check in facilities (and fast security privileges) are only available for Korean Air, Asiana and Jeju Air passengers there.

If you are staying in Gangnam, the City Air Terminal at COEX offers the same services (with limousine bus transportation instead of train) for MANY more airlines. From the CALT website, these are the airlines that offer check in at CALT:

Korean Air, Asiana Airlines, Jeju Air, Qatar Airways, Singapore Airlines,
Thai Airways, Air Canada, Philippine Airlines, MIAT Mongolian Airlines,
China Eastern Airlines, Shanghai Airlines, China Southern Airlines, Japan Airlines,
KLM Royal Dutch Airlines, Delta Airlines, United Airlines, Air France