The save file for this game is hidden pretty well so here's a quick guide on how to access them.

The path to the save file should be one of these:

C:\Users\Username\AppData\Roaming\Arrowhead\Gauntlet

C:\Users\Username\AppData\Roaming\Gauntlet

After you locate the save files, simply rename the original saves folder if you wish to keep your save or delete it to start a new game.

Also, to prevent your old save from being loaded by steam automatically, you need to disable steam cloud for Gauntlet within steam

That's all!