What world downloads are
World downloads are save files from other players that you can play yourself. Adventure maps with stories and puzzles, parkour challenges, PvP arenas, survival islands, amazing builds to explore, and complex redstone machines. People have been sharing these for years.
Installing a world
Download the world file (usually a .zip). Right-click your instance in GDLauncher, select More options → Open folder. Navigate into the instance folder, then the saves folder, and extract the .zip there.
After extracting, you should have a folder with the world's name containing level.dat, a region folder, and possibly a data folder. Make sure level.dat is directly inside the world folder, not nested in another subfolder (a common issue with some downloads).
Launch Minecraft, click Singleplayer, and your new world should be in the list.
Version compatibility
Worlds can be opened in newer Minecraft versions and will upgrade automatically. Going the other way (newer world in older version) can cause problems or data loss.
Check what version the world was made for. If it says 1.19, you can play it on 1.19 or later. Playing on 1.18 might break things.
Modded worlds
Some worlds require specific mods. The download page usually lists requirements. Without the right mods, modded blocks will disappear or turn into air. Make sure you have everything installed before loading.
Troubleshooting
World not appearing in the list? Make sure you actually extracted the zip, not just copied it. Check that level.dat is in the right place. The most common mistake is having the world nested inside an extra folder (like WorldName/WorldName/level.dat instead of WorldName/level.dat).
World won't load? It might be from a newer Minecraft version than you're running. Or it needs mods you don't have. Try downloading again if the file seems corrupted.
Missing blocks? The world probably needs mods you don't have installed. Check the download page for requirements.