How many Solar thermal panels for 1.2 therms a day?
You would need to know the output of the panels for the place where you live (amount of sun), any conversion efficiency (like to 120/240 volts) and the efficiency of the gas furnace you are using now and of the electric unit you would be using afterward. So you would need several sets of numbers from various vendors and have to allow for reduced sun power in the winter when you use the most gas.
It is most likely that an honest vendor would tell you that you are unlikely to eliminate your gas heating bill, but you might reduce it considerably by having dual gas electric in some form. If your utility does buy back of power, it might be worth it to have a bunch of solar power, feeding to the grid on hot sunny days and borrowing from the grid to heat on cold gray days.
How efficient is it to heat a 31,000 gal pool with a 4 panel, 105 gal tank solar thermal system?
I do not think it is efficient to heat such pool using the 105 gal solar tank, the amount of raising the heat by emptying this tank will not be magnificent and majority of the gained heat will dissipate at night , possibly during the day time only two times the 105 gal tank can be discharged into the pool