No, the site you are looking at is wrong, there never was an FGP. All 1930 and 1931 Scout motor numbers were EG for 37" and EGP for 45", just as all 28 and 29 motor numbers were either DG or DGP. The letters were carried over for two years, the numbers were consecutive, so, for example, with 1930 it is generally accepted that the EGP 3079 was the motor number of a 1930 style model and EGP 3081 was the first "1931" model with the new rear brake set up and hence changes to the rear right hand frame lug. 

The Indian motor and frame numbering system changed again for 1932.

There were no strict definitions of year model, rolling  changes to equipment were introduced as the factory ran out of old parts and brought in updates. In some cases, older style models continued to be sold overseas as new models after they were not available in the US.

Again, as an example, the factory tended to announce  new models in the fall of the previous year, for the main motorcycle shows, so around August 1927, the new style motor was announced as a 1928 model, in a short frame style chassis. These bikes had a motor number of CG or CGP. The 101 Series was also a 1928 model, but was not announced until early 1928 and  brought into production until March 1928, with motor numbering DG or DGP. Same motor, different frame and rolling chassis. 

So, effectively, in October 1927, you could have bought a 1927 pedestal mount scout 45, with a motor number BGP, or a 1928 model thru bolt motor with a CGP engine number. Both of these would have been designated as a Model GEP Scout, either a Police Special or a Sport Solo, and both are generally referred to now as short frames. Early 1928 Factory literature only lists the short frame GE and GEP models, while later 1928 factory literature announces the new 101 series for sale alongside the old short frame series.

It can be a little confusing. However, you definitively have a 1931 101 frame.