Another possibility: (common for such puzzles):
If we try of dghlcmugaxmgbm8gag9wzq (not standard Base64 length, but padding may be missing), it doesn't decode cleanly. dghlcmugaxmgbm8gag9wzq
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. Another possibility: (common for such puzzles): If we
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. but padding may be missing)
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 .