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My experience so far is that when I tried to run the game, galaxy loads. Does this mean that GOG Galaxy is really an embedded launcher for all game executables in the future? And this would enable multiplayer? I'm confused about where we're going with this, but so far I have nothing but positive impressions.
You can launch AvP without the Galaxy client, if you just use the non-Galaxy shortcut in your start menu (or run it directly from the game directory). The reason they only put the Galaxy shortcut on the desktop is that this is intended as a test of the multiplayer functionality of Galaxy.