I believe it stands for Phased Plasma Gun.

Re: the jump-ship scenario described just above...since the "exit ramp"
jump-gate is usually (but not always) under the control of a nearby base, as
B5 is tied into its local gate, it could *probably* contact the local base and
request activation of the gate. But it's a good point to consider, and I want
to dwell on that for a while before I lock it down.

It opens up some interesting possibilities.

jms