advowson: I regret to report total failure here. Tapping the Start Screen shortcut Aliens vs Predator Classic 2000 - Galaxy Client produces no apparent reaction. I can see the process running (C:\users\me\Temp\GalaxyUpdater.exe /clientPath=C:\Program Files\GalaxyClient\GalaxyClient.exe /command=launch /gameId=1207665883 /installationPath=C:\Program Files\GalaxyClient /path=C:\GOG Games\AvP Classic /waitForPreviousInstance), but nothing appears. Network activity shows repeated attempts, without an answer, to resolve galaxy-log.gog.com. and cfg.gog.com.. From the perspective of the naive user, it looks like the process failed since no window ever appears. I started writing this forum post and left the GalaxyClient running in the background. After ~14 minutes, I was interrupted by a dialog which reads:

Galaxy Updater failed

Galaxy Updater encountered an error and could not continue: Updater could not download update metadata.

Dismissing the dialog works, and I once again see no windows related to GOG. I see more network lookups for the two domains mentioned above, and it looks like the updater client restarted itself with all the same options, under a new PID. After several more minutes, I got a window labeled "GOG Launcher 0.1.0.456" and an option to start the game through the launcher. The status text says OFFLINE, so I do not expect multiplayer to work. Indeed, it does not: "Failed to initialize." is shown when I choose multiplayer in the menu.

Failing to get anything out of the GalaxyClient, I decided to explore single player. That did not go well, largely due to a complete lack of documentation. I never knew of this game before the GOG beta, so everything I know comes from the forums and from blind experiments in game. I saw no manual or other documents listed in the game library, so I installed the game blind, hoping that a PDF would be among the game assets set down by the installer. No such luck. I then hoped that the single player campaign would include a tutorial phase to walk me through operating the game. Again, no such luck. Finally, I give up looking for guidance and set off for blind experiments.

First run: play the Alien campaign. I spent ~5 minutes unable to find a way out of the starting area, got frustrated, quit, and started browsing the forum. By chance, a forum post mentioned that the alien can climb walls, but only if you keep it crouched while doing so.
Second run: Alien campaign, now with the ability to climb walls (by default: hold down left-control). Climbing the walls does not seem to help, so I still have no idea where I'm going or what I'm doing, so back to the forum. Someone mentions a grating in the ceiling, so back to the game to start exploring the ceilings. I found a tunnel that led me to a new room with a waterfall. Upon touching the waterfall, I was instantly kicked to a defeat screen. That sucked, because I don't even understand why I was defeated.
Third run: Alien campaign, made it back to the waterfall room after lots of blind casting about. A map would be really handy about now, even if only an in-game map showing where I have been. I disabled the alien night vision and now the waterfall is more of an orange color. At this point, I suspect it killed me the first time because it's actually lava, not water. I resume exploring and somehow managed to drop myself back into the starting chambers, at which point I got frustrated with the lack of a map and quit. I never even found a chance to engage in combat.
Fourth run: Skirmish mode. Alien is not available here, so I use a Predator since forum posts say they are overpowered and I want to explore without getting instantly killed. I played around with the Predator vision options a bit, toggled the cloak a few times, then set out. The vision options put some nice color filters on the scene, but I do not know which of them I should use or what they do. I promptly fell down a shaft and was attacked by a computer-controlled Alien. I tried to fight back, but seemed not to be attacking. The alien seemed to attack me, but I cannot tell if it was doing any damage because I have no idea what the glyphs on the screen are trying to tell me.

I see forum posts from users who got everything working and had a great time with it. My experience is that, if you do not have fond memories of how the game works, trying to learn it as part of the beta will only lead to frustration.