The Global mm-VLBI Array (GMVA) is a network of 14 3 mm and 7 mm capable telescopes spanning Europe and the United States, with planned extensions to Asia. The array is capable of sensitive maps with angular resolution often exceeding 50 μas. Using the GMVA, a large sample of prominent y-ray blazars have been observed approximately 6 monthly from later 2008 until now. Combining 3 mm maps from the GMVA with near-in-time 7 mm maps from the VLBA-BU-BLAZAR program and 2 cm maps from the MOJAVE program, we determine the sub-pc morphology and high frequency spectral structure of y-ray blazars. The magnetic field strength can be estimated at different locations along the jet under the assumption of equipartition between magnetic field and relativistic particle energies. Making assumptions on the jet magnetic field configuration (e.g. poloidal or toroidal), we can estimate the separation of the mm-wave "core" and the jet base, and estimate the strength of the magnetic field there. The results of this analysis show that on average, the magnetic field strength decreases with a power-law B ∞= r^-n, n = 0.3 ± 0.2. This suggests that on average, the mm-wave "core" is ∼ 1-3 pc downstream of the de-projected jet apex and that the magnetic field strength is of the order B_apex ∼ 5-20 kG, broadly consistent with the predictions of magnetic jet launching (e.g. via magnetically arrested disks (MAD)).