BMW has gone loco with their naming conventions of late, especially with their cars. it used to be simple and clear. "325" meant a 3 series car with a 2.5l engine. now...who knows what they are.
for bikes, first letter was engine configuration. numbers after letter engine size. letters after that the style of bike.
K - longitudinal cylinders
R - horizontal opposed cylinders.
but then....they kept the "K" and put a transverse slant 4 in a bike...the K1200/1300. oops. broke the template. changed their minds a few years later and called the Slant 4's "S" bikes...the S1000RR, R and XR, and dropped the "K" designation altogether.
F - the transverse slant 2cyl rotax engine bikes F800GT, G, GS
G - the single cyl micro bikes G310R, GS