How do I make use textures from other mods/texture packs?

When I tried using textures from the gothic texture pack to mod final doom, when I booted the map up, it just showed up as a red texture. How do I fix this?

You cannot simply load Gothictx with Final Doom IWAD in classic non-ZDoom ports, because these ports don't support stacking TEXTURE1 entries. Only the last TEXTURE1 will be loaded. That's why the last loaded TEXTURE1 must contain all texture definitions, including the IWAD ones. Gothictx was made for Doom 2, not Final Doom, so that you need to create your own Final Doom compatible TEXTURE1 - the guide is below:

Open your wad in SLADE3 and delete all texture graphics, TEXTURE1 and PNAMES lumps. Now open the IWAD you're using to play the map. Copy TEXTURE1 and PNAMES lumps from the IWAD and paste them into your wad. Now open TEXTURE1 lump by double click. You will see a list of texture entries. Now open Gothixtx.wad in a new tab, double click its TEXTURE1 lump and see the list of gothic textures. Select all Gothictx-specific textures - not the IWAD ones at the beginning of the list, only the new custom ones. Copy them, then go back to the tab with your wad's TEXTURE1, and paste the textures in there. Close TEXTURE1 and confirm to save changes to TEXTURE1, then save the wad again. Now check the contents of your wad. All Gothictx patches should be between P_START and P_END (or PP_START and PP_END) markers now. If some of them are not, move them so that they will be. Finally, copy the flats from Gothictx into your wad, place them between FF_START and F_END markers (yes, double F in FF_START, single F in F_END). Save your wad and you're done.

If you don't want the Gothic textures to be inside your wad - no problem! Follow the guide above, then delete all of the patches and flats inside your wad, and only keep the TEXTURE1 and PNAMES lumps. Then load the wads in this order: Gothictx.wad first, your wad second. The TEXTURE1 in your wad will be loaded, and it will work with the patches from Gothictx properly.