Question

Is there a better way to estimate or find the exact value 
of thickness gradient since we only have two thickness lines 
to work with at any given time period? For example, is 
thickness gradient analogous to height gradient?


Answer

The value of the thickness gradient has already been 
incorporated by GARP in its calculation and display of the
nearly LTHAD fields. There's no way to back out the magnitude
of the thickness gradient from these maps. You only have the
limited 1000-500 hPa Thickness maps (two contour levels only)
from which to make a crude thickness gradient estimate.

Thickness gradient is only analogous to height gradient in
an equivalent barotropic environment. You have to decide if
the environment is similar enough to an equivalent barotropic
environment to make such an analogy valid.