Research on teachers' technological pedagogical content knowledge (TPACK) has been burgeoning recently. Yet, little is known about how teachers integrate AI tools such as ChatGPT in language teaching. This preliminary qualitative study investigates the exploration and incorporation of ChatGPT in language teaching by graduate student instructors (GSIs). By analyzing data from questionnaires, focus group interviews, screenshots of interactions with ChatGPT, and participants' lesson plans, this study shows how instructors develop their knowledge about ChatGPT and mobilize content and pedagogy knowledge to enact technology integration. Findings reveal that GSIs adopted various strategies when exploring the affordances of ChatGPT. Furthermore, while GSIs form positive perceptions of ChatGPT affordances, negative perceptions pertain to its limited capacity to process the Chinese language. Lastly, GSIs drew on various aspects of TPACK to design lessons, among which content knowledge and its interplay with technology seem to be prominent.