Here is a list of articles that describe techniques for forging helical gears:
J. Cai, et al, "Alternative die designs in net-shape forging of gears", Journal of Materials Processing Technology 150 (2004) 48-55
T. Dean, "Precision forging", Proceedings of the IMechE Vol 214, 2000, Part C (Journal of Mechanical Engineering Science), 113-126
T. Dean, "Net shape forming of gears", Materials and Design 21 (2000) 271-278
Z. Hu, et al, "Properties and performance of forged gear forms", Proceedings of the International Conference on Mechanical Transmissions, 2001, 123-127
T. Dean, et al, "Net-shape forged gears - The state of the art", Proceedings of the International Conference on Mechanical Transmissions, 2001, 25-31
Y. Park, et al, "Finite element analysis for precision cold forging of helical gear using recurrent boundary conditions", Proceedings of the IMechE Vol 212, 1998, Part B (Journal of Engineering Manufacture), 231-240
E. Doege, et al, "Closed die technologies for hot forging", Journal of Materials Processing Technology 98 (2000) 165-170
V. Vazquez and T. Altan, "New concepts in die design - physical and computer modeling applications", Journal of Materials Processing Technology 98 (2000) 212-223
The first 5 papers are from the same research group at the University of Birmingham in the UK, so there is some overlap in the content, but Prof. Dean is one of the foremost experts on this subject. Prof. Doege and Prof. Altan are two of the other leading experts in this type of work. Some of the papers have a lot of theoretical discussion and computer modeling, but they all show examples of tool designs capable of forming/forging helical gear teeth. Best of luck on your endeavor.