From Gyeongju again to Seoul, the course of action is really very similar. Visit the Gyeongju Specific bus terminal, and board your bus to Seoul from there. Scan your ticket QR code when boarding the bus. If you need information about general public transportation in Gyeongju Look at this https://gyeongjuoffice53187.blogaritma.com/30337613/the-smart-trick-of-gyeongju-office-usage-price-guide-that-nobody-is-discussing