Configuring it didn't fix the spacebar issue for me. I have no way of opening doors or using switches.

Use the "F" key instead

Don't use the same controls as the most recent Shoot 'Em Up.

Press 2) Game DOS Settings > Configure Controller > Keyboard > to change USE=F to SPACE highlight USE=F and press Enter then Space then press F10 to accept (F10 is very important or the game will still use F) and then Save parameters and start game

you should now have the spacebar as the use key

If you are playing Heretic or the first Hexen game, use a sourceport, like GZDoom, Chocolate Heretic, or Chocolate Hexen, instead.

GoG look like they've configured this to run using WASD and "F" to open out of the box; OP is correct in that you need to check the controller settings in the config file.

Personally, I'd rather have had the classic control experience out of the box, but I'd imagine that people have got so used to WASD and mouse that it's second nature to them now (although mouse turning without proper mouselook is weird).

My recommendation is that if you want to run with WASD and mouselook, use a sourceport like GZDOOM, otherwise re-bind to the original controls and play it the way it was released.

Get GZDoom. It lets you modify all of your hotkeys in-game. It also provides a lot of other features, such as mouselook options and the ability to increase resolution if you want.

It's super confusing because the controls listed in the help screen are completely wrong.

Yes - it's annoying because GoG have rebound the defaults so that it plays the same as the Doom 3 BFG edition ports of Doom and Doom II.

If you don't want to use a sourceport, change them back in the Configuration utility (and at the same time, set the General MIDI sound as well - so much better - you may also want a Virtual MIDI device and some soundfonts)

If the controls are not revamped, the defaults GOG version controls seem to be F or double right click for open doors/switches and ENTER to use inventory object