"Mucking" is another term for "messing with."
"G" is short for "G-man" which was slang for Government Man, which is again slang for special agents of the US Federal government law enforcement bureaus and agencies.
Essentially it means, " You're messing with a special agent"
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It can mean anything for example, 2g + g = 3g
What does M.g.B. Mean?
It's 88 Keys on a Grand Piano
K.P.G. = Kilometers Per Gallon it means plumb gold
There are a few geometric terms that start with the letter G including great circle, glide, and golden ratio. Another example is golden mean.