[dmoy]

I think GP means flag as in flag state - ocean vessels are typically to some country. In this sense, nearly all submarines are flagged - US navy, Russian navy, whatever.

Not as in a literal flag flying on the submarine. (Though they do fly flags near ports and such)

[AngryData]

Their is an entire DIY submarine community around the Caribbean and those guys aren't part of any navy or military and don't fly flags.