Xmodmap definitions for greek letters when typing AltGr+<letter>. Not all greek letters included. Can add more by getting letter names from https://www.cl.cam.ac.uk/~mgk25/ucs/keysyms.txt

greekLettersXmodmap

keycode  26 = e E e E Greek_epsilon Greek_EPSILON Greek_epsilon Greek_EPSILON
keycode  27 = r R r R Greek_rho Greek_RHO Greek_rho Greek_RHO
keycode  28 = t T t T Greek_theta Greek_THETA Greek_theta Greek_THETA
keycode  33 = p P p P Greek_pi Greek_PI Greek_pi Greek_PI
keycode  38 = a A a A Greek_alpha Greek_ALPHA Greek_alpha Greek_ALPHA
keycode  39 = s S s S Greek_sigma Greek_SIGMA Greek_sigma Greek_SIGMA
keycode  40 = d D d D Greek_delta Greek_DELTA Greek_delta Greek_DELTA
keycode  41 = f F f F Greek_phi Greek_PHI Greek_phi Greek_PHI
keycode  42 = g G g G Greek_gamma  Greek_GAMMA Greek_gamma  Greek_GAMMA
keycode  56 = b B b B Greek_beta Greek_BETA Greek_beta Greek_BETA
keycode  57 = n N Greek_nu Greek_NU Greek_nu Greek_NU
keycode  58 = m M m M Greek_mu Greek_MU Greek_mu Greek_MU
keycode  32 = o O o O Greek_omega Greek_OMEGA Greek_omega Greek_OMEGA