Graduate Teaching Assistants (GTAs) are key partners in the education of undergraduate students. In large-enrollment intro physics classes, students spend roughly half of their in-class hours in labs and recitations under the supervision of GTAs. Since GTAs can have a large impact on their students' learning, it is important to provide them with appropriate preparation for teaching. But GTAs are also students themselves - they have many demands on their time, and not all of them want to become professors after grad school. Therefore, it is crucial that GTA preparation not be a burden but rather be fully integrated into their professional development. The School of Physics at Georgia Tech has been offering a GTA prep course for first-year Ph.D. students since 2013. The majority of these first-time GTAs have no prior teaching experience but consider teaching to be an important part of their professional development as physicists. Through a cycle of implementation and revision, and guided by the 3P Framework we developed (Pedagogy, Physics, Professional Development), the course has evolved into a robust and comprehensive professional development program that is well-received by physics graduate students. We assessed the effectiveness of the course with a combination of surveys, pre/post tests, and student evaluations. We found that GTAs feel better prepared for teaching after going through the Orientation. GTAs consider most useful the course activities in which they can practice and get feedback on their teaching ("Microteaching", "Lab Simulation") and the lessons in which we discuss the pedagogical content knowledge necessary to teach intro physics labs and recitations ("Teaching Physics"). GTAs who participate in the GTA prep course adopt more learner-centered teaching approaches and increase their pedagogical knowledge. They also receive higher end-of-semester student evaluations than GTAs whose first teaching experience predated the establishment of the GTA prep.