GUMS is described in Robin et al. 2012, A&A 543, 100 
The details of the implementation are described in the Universe Model Overview, GAIA-C2-TN-LAOB-AR-004. 

2MASS (J,H,K) and SDSS (u,g,r,i,z) photometry have been added the official GUMS 18. Those have been computed from the Basel models (Lejeune et al, 1997, Astron. Astrophys. Suppl. Ser. 125, 229-246) as a function of Teff, logg and metallicity. For white dwarfs, models of Tremblay P.-E. and Bergeron P. 2009 ApJ 696 1755 have been used.