Given the symmetry, I’d guess the is that after applying Atbash, you get welcome to the puzzle or similar, but my quick attempt didn't yield that.
Since the instruction says "helpful piece" — if this is from a puzzle, the answer might be a word or phrase like "helpful piece" itself, meaning the decoded string is helpful piece . dghlcmugaxmgbm8gag9wzq
Another possibility: (common for such puzzles): Given the symmetry, I’d guess the is that
If you intended a simple Caesar shift: Try shift of 5: d(3)→i(8), g(6)→l(11), h(7)→m(12), l(11)→q(16), etc. — but that doesn't produce English either. Given the symmetry