In the past, when I hosted many of these games at LAN parties, they would work perfectly fine under Windows 7. It wasn't until I updated most of my desktops to Windows 10 that nearly all of my GOG games began having issues with LAN. Some games work perfectly fine using direct connect (manually entering the IP), but for the games that don't let you use the internet game menus because GameSpy is down is where the bigger issue is at. The only game I have a known workaround for is Hidden & Dangerous 2 which as a Windows 10 specific fix via registry edit. Has anybody got some tips & tricks for my issue?

EDIT:
I've come to a conclusion. If you cannot get LAN to work under normal circumstances, use direct IP. If the game does not allow an option for direct IP because it's locked behind the GameSpy menu, maybe a GameSpy alternative* solution could do the trick. Otherwise, do not use Windows 10 to play on a LAN for those games; buy a cheap 7,200RPM HDD specifically for Windows XP/7 in that case.

Alternate Solutions for dead GameSpy games [for non-LAN]
ZeroTier - Low latency and a cap of 100 users on free accounts.
333 Networks - A community maintained master server alternative, typically for games running on the Unreal engine family. [some games require per-game configurations, other games might work universally with a 'hosts' file edit.]
QTracker - great LAN menu alternative but only for the games it supports.
GameRanger - Best left as a last resort but I have had great experiences with the games it supports. I suggest only playing with high bandwidth users though.

LAN = Local
Gamespy is an internet service (now defunct) - how does that relate to LAN functionality? And why would Windows 7 games that rely on Gamespy for multiplayer still work?

On Windows 10, I am relying on the online menu to manually enter the '192.168.1.X' IP addresses of locally hosting computers to establish a handshake between machines because the LAN menus refuse to show any games. Before on Windows 7, I had zero issues using the LAN menus.

When I say "before", I mean there were zero issues hosting LAN sessions.

That doesn't answer my question. When are you talking about? Was that before May 31, 2014?

The correlation to GameSpy is that some games prevent you from looking at the online menu which has the 'Direct IP' feature. Some games don't have that in the LAN menu. When my computers had Windows 7 installed. It seems Windows 10 probably expects different network environments or condition compared to Windows 7 which is why LAN isn't working like it used to.

If the multiplayer aspect of a game needs the Gamespy service to be online (it isn't anymore), then it won't work in Windows 7 anymore either.

Some games got an official or community patch to still play multiplayer locally without Gamespy, others didn't. That's multiplayer DRM for you.

You're not reading.

The games I am trying to play in a LAN used to work perfectly fine under Windows 7. Now that my computers have Windows 10, I am no longer able to play LAN without having to take extra steps; manually entering local IP addresses for example. We can just play over the internet but it defeats the purpose and introduces higher ping which is bad for older games.

The point in: if a game NEEDS an online service (like Gamespy) for LAN play, then LAN play won't work anymore on ANY OS (unless a patch exists).

Usually (in the past), LAN functionality means: no internet connectivity needed. An online service might be a lazy way to add some convenience - but essentially it is DRM.

The purpose of this thread is to figure out if all I need to do is make some configurations to my Windows 10 network in order to get LAN working normally again. I'm doing tons of troubleshooting on my end and will reply with any breakthroughs I come across that would be of use to other Windows 10 users prospecting to enjoy LAN normally again. Unless you have any information on that, I would appreciate it if you stop posting in this thread.

Why don't you check the subforums here on the specific games you want to run. I doubt that a general solution exists for "any" game.

Found absolutely no solutions for the games I'm trying to get to work apart from Hidden & Dangerous 2.

Try here: https://pcgamingwiki.com/wiki/Home
Tell me the name of one of these games; maybe I can look up more for you about the nature of your issues.

I looked up each individual game. Then again, I might be the only soul trying to get these games to play in a LAN on Windows 10 which is why no information is available.
Those closest to a solution was general networking information for Sniper Elite.

https://steamcommunity.com/app/3700/discussions/0/154642901285761088/