RGL = Remington Game Load...,
RGL is a term often used in 16ga reloading, since it accounts for 99% of the Remington 16ga hulls you'll encounter. Also, it is unique in it's internal capacity which is less than all the other common hulls... (Fed, Fiocchi, Win, Kent, RST, Cheddite, etc.) The RGL is often derided by some 16ga reloaders who can't appreciate that it is the ONLY hull capable of loading from 5/8 ounce all the way up to 1& 1/8 ounce WITHOUT requiring "fillers" to get good crimps.
Not sure where you're seeing "RTL" but it probably alludes to Remington Target Load, since the 12ga hulls are all interchangeable...Premier, Blue Magic, STS, even Gun Clubs.