Is there one central place where I can find DEMOs on GOG? If not, then what is the best way to find them?

https://www.gog.com/en/games?tags=demo

Meaning, go to browse all games and check the "demo" tag. Unfortunately, I can't promise you this will list them all, because as has been established many times the tags around here tend to be somewhat borked.

Or this: https://www.gog.com/en/games?priceRange=0,0&hideDLCs=true

It selects free items and excludes DLC, so it includes free games as well, but most matches are demos.

Edit: also, this thread: https://www.gog.com/forum/general/the_list_of_demos_on_gogcom