Kind of a dumb comparison.
The PG was an elite military unit. The SS is civilian.
The PG had tremendous political clout, to the point they often chose the next emperor and forced him down the throat of the Senate. The SS has none.
The PG put down riots and provided more or less police services in the capital. The SS does not.
What do you think the two organizations have in common, other than the (essential) job of protecting a head of state?
Hmmmmmmm, can someone tell me how Caligula met his end? I seem to have forgotten that one.
*Whistles with eyes upturned*