So I want to make a cheap gaming build, and since more and more games are coming out on Linux, I'm going to try and start with it to save a hundred bucks. My question is: are there any mobos/cpu/gpus/wireless adapters/anything else that won't work with Linux?
That is a very difficult question to answer.
Motherboards, CPU's, RAM and Graphics Cards are all Linux Compatible. So are SSD/HDD's and Opticals (not BLU-Ray drives I suspect).
Wireless Network Interface Cards possibly not, as they require drivers and there may not be a Linux version.
Basically anything that requires a software to run or drivers is possibly Linux incompatible. However on the the more "core" components of the rig there will be Linux drivers.
Also there is a saying among the tech community, "Linux is only free if your time is worthless". I have a a working installation of Ubuntu on one of my HDD's that I have never used, quite simply because its too much effort to understand how to make everything work and I cant be bothered.
In my experience (running Ubuntu as my OS on three systems) I have not had a problem with motherboards or CPUs being incompatible. It's the other components that tend to give you problems--remote controls, mice, cameras, etc. Still, 99% of stuff out there works. If you don't buy anything unusual it will most likely work fine. You could also do a search and copy someone else's working build.
I disagree with the "Linux is only free..." comment. You can encounter just as many problems in Windows/Mac OS, especially from viruses/malware. If you have ever had to reinstall Windows due to a virus attack, then you know what I mean. I eventually made the switch because I was fed up with virus attacks (my wife just can't stop clicking on those attachments). Linux has its issues, but one huge advantage is that you don't have to spend time or money worrying about viruses.
Same with system bloat...in Linux you do not have the layers and layers of background processes that you inevitably have in Windows. Even my 7 yr old Dell single core Celeron boots up in about 20 seconds.