After looking over some of the content of the web site, it appears to take a position on a controversy within evangelicalism --between believing in Jesus as Savior as distinguished from believing in Jesus and Savior and Lord (the latter sometimes being termed “Lordship Salvation”). GTS takes the former position.
It’s true that some of the leadership was trained at Dallas Theological Seminary, a well known and widely respected evangelical seminary, and probably THE academic center of dispensational theology. I’m a graduate of DTS, and attended there from 1982 to 1986. Zane Hodges was my professor in New Testament Greek. Dr. Arthur Farstad was also a professor at DTS, and I knew him well. He was also the executive editor of the New King James Bible, and a careful New Testament scholar. In fact, I knew both Hodges and Farstad to be fine, godly men, but their views, as far as I know, are not part of any official position of Dallas Theological Seminary. Dr. Farstad was Plymouth Brethren in his church affiliation, and a small congregation met in his house for many years. I’m not sure about Hodges, but I think he would consider himself a nondenominational evangelical.
I don’t know that I would call the positions of Grace Theological Society cultic, but they do seem to have taken the Reformation principles of sola scriptura, sola fide, sola gratia, and sola Christos to the furtherest extreme.