In our plant, we number equipment with a unit number followed by a letter that tells us what type of equipment it is followed by a sequential number just to distinguish between different services followed by a letter. The first letter would be P for pump, E for exchanger, V for vessel, etc. The last letter would be A, B, C, etc. to show that this was a group of identical equipment typically installed in parallel. For pumps, this would usually mean A is the main and B is the spare.
Today I will be doing some work on 25P-20A. This breaks down to 25 unit (our #3 crude unit), Pump #20 happens to be the heavy vacuum gas oil service, and A means I am dealing with one pump out of an identical set. In this case, there is also a B pump. A is motor driven and runs normally. B is driven by a steam turbine and is typically on standby as the spare.
Our sister refinery in Texas uses a system that looks like the one you are asking about. G refers to general equipment. GA would be a general purpose pump. I don't know what GB or GC would refer to. It is a very cryptic system that I don't care for.
These are the two main equipment numbering systems that I have been exposed to. Ours seems to make more intuitive sense. But, I suppose, once you learn the system, it really doesn’t matter much.
Johnny Pellin