I had it too (same shape) and it was definitely the hardest game, though I did beat it once with the wizard. While SMB 3, Zelda 2, and the Castlevanias were my favorites and clearly in another league, Gauntlet was outstanding to me at the time, especially the music.
Yeah Gauntlet holds a special place in my heart, which got me to buy the... remake? that came out on PC a few years ago. I respect a game that is savage and willing to stick to that. Though a fair bit of how insane it was may be tied to the arcade roots where they wanted you to keep feeding in quarters.
I remember this! It was a black cart, and it stood out like a sore thumb.