Workshop maps
1.Subscribe your desired map.
2.Wait for download to finish.
3.Open your computer and got to the disc on which you have installed steam.
4.Follow this path Program Files/steam/steamapps/workshop/content/440
5.Find your map and copy map BSP file.
6.Go back to steamapps and follow this path common/Team Fortress 2/tf/maps
7.Place it in a map folder and you are done.Your workshop map is now in TF2 and GMOD.

Gamebanana maps
1.Download desired map
2.Wait for download to finish
3.Open downloaded file and copy BSP file(depends on map)
4.Follow this path Program Files/Steam/Steamapps/Common/Team Fortress 2/tf/maps
5.Put BSP file in map folder and you are done.
Some gamebanana maps have more than BSP file.For example Cp_Graygravel has files for sound,models and materials folders.They shoul be put in My custom stuff folder or materials,sound and models folders.(I recommend to put in my custom stuff,because it's much easier)
You are probably asking Why we are porting maps to TF2 instead of GMOD?
Answer is simple.When we install Team Fortress 2 or other Source game Gmod will scan it to gather props,ragdolls and maps.When you put custom map in maps folder it will be read it by GMOD as default map.Custom maps have working physics,but props on this map are not moveable.This method doesn't work with custom skins.