system
A chat on subreddit /r/pcmasterrace
human
What do you mean by "force the setting"?  
How does it work?  
Also I guess I could go for just freesync since they would all be using DP but... can g-sync and freesync work together for my main PC + side monitor?
gpt
What they are telling you is not quite right, some monitors with freesync work 100% there is a list on the Nvidia site that will tell you if your monitor is compatible, the Nvidia control panel will also show the option.

Some other monitors do not work at all, it's due to the Hz range the monitor supports while using freesync, for instance gsync hertz range is, 30-144hz, freesync range is 48-144hz, and then there are differences in the panel and manufacturer.

Note: you can still force the gsync setting to ON, but you may see artifacts, odd brightness effects etc depending on your monitor