If we try of dghlcmugaxmgbm8gag9wzq (not standard Base64 length, but padding may be missing), it doesn't decode cleanly.
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): If we try of dghlcmugaxmgbm8gag9wzq (not standard Base64
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. — but that doesn't produce English either
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.
If we reverse dghlcmugaxmgbm8gag9wzq and try Atbash, we might get it, but manually it's tedious.
The string dghlcmugaxmgbm8gag9wzq appears to be encoded, possibly with a cipher like Caesar cipher, Atbash, or Base64.