A GMPC is not an NPC. It's a Player Character run by the DM. It is the DM's personal mouthpiece and proxy, just like the characters are for the players. The party has no choice but to incorperate the GMPC, just like it does any other PC.
None of those apply to an NPC.
To me they're the same thing. An adventuring NPC in the party is every bit as much a part of it as are each of the PCs - if for no other reason than people in the game world don't go around with little stickers on their foreheads that say PC or NPC and thus the PCs have no reason to treat the NPC any differently - and thus is allowed to have character, opinions, preferences, etc. and maybe even the occasional bright idea.
The party have every bit as much choice on incorporating a party NPC as they do a PC.
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Just for kicks, after posting my earlier thoughts above as to why NPCs end up in the party, I went back through my game logs and started running the numbers. Still in progress, but early returns indicate that over time by far the biggest driver behind having NPC adventurers in the party has been the players-as-PCs themselves!
Straight-up proactive recruitment of new full members is the greatest cause of party NPCs, and it's not even close.
A significant subset of this is recruitment of henches who, quite often, later get promoted to full members.
Another big NPC generator is rescued friendly prisoners who join the party to help finish the adventure and then sometimes stick around.
Charmed or captured enemies who join the party not of their own volition is another (usually short-term) NPC generator.
Other less-common party NPC generators include module-driven plot devices, local guides, field observers, PC companions, merging with another entire party in the field, ex-foes switching sides, etc.
Of all of these, only the plot devices and the field observers can really be seen as DM-forced. All the others are either directly player-driven or due to the players' in-character choice(s) and-or action(s).