This study examined the effects of teacher professional development in Global Englishes Language Teaching (GELT) for 60 in-service English teachers in Thailand, covering both primary and secondary schools. Over four months, the professional development included a series of transformative activities designed to encourage teachers to rethink their existing beliefs about English and English Language Teaching (ELT), as well as to adjust their teaching methods to align with the GELT framework for preparing students to communicate in English in a global context. Using mixed-methods research, the findings indicated a slight improvement in teachers’ attitudes toward GELT and a moderate enhancement in their skills to implement it in the classroom. Furthermore, teachers expressed greater confidence in teaching English, as the GELT framework empowers them to take ownership of the language. Because participants entered the program with a solid foundation in GELT knowledge, their overall understanding remained unchanged. The discussion explores the concept of trans-speakerism, highlighting how the GELT framework influences teachers’ identities, and their perception of agency in being recognized as legitimate Global Teachers of Englishes (GTEs).