Question on a notice for a horse race, 1838

Looking for some help on understanding a few parts of this notice



Question on the second paragraph, 2 line

I presume 'Arabs' means Arabian horses, 2 miles the length of the race, 9 stone for 'jockey' weight perhaps? (9 stone equals 126 lbs), 20 G.M. Entrance P.P.

Does anyone know what G.M. might stand for? or 'Entrance P.P.

Thanks!