Enter chord Jazzfont

If all else fails use it's Enharmonic equivalent - Cm7b5 (half diminished)

Without knowing your reasons for asking, I suspect that Cm7b5 (or the various other spellings like Cmi7b5 or C-7b5 depending on the chord symbol style you have selected) really is what you mean. Or more particularly, Cm7b5/Gb if the Gb in the bass is important. That's a very standard / common chord symbol. So is Ebm6/Gb, which also works out to the same thing. Gb6b5, on the other hand, is virtually unheard of - probably no musician alive has ever seen it, and it would probably leave them quite confused, although once they did the math and worked it out, they'd get it right I'm sure.

Anyhow, in the current released versions of MuseScore, you need to type chord symbols it can recognize, and it will only recognize them if they are relatively "normal". For 2.0 Beta/Nightly builds, you can actually type unexpected things like Gb6b5 and have them recognized. Still, unless you have a very special reason, just go with a more standard name.