Is there a standard symbol for gain margin?
Based on the comments, and on the new sensitivity to the topic I always get from submitting a StackExchange question, I believe the best answer is: \$GM\$

I've noticed this usage several times since reading the comments I received in response, and have encountered no alternative symbols or abbreviations.

I have noticed a tendency for it to appear with \$PM\$ as the expression for phase margin, as @Chu mentioned, and definitely agree with @LvW's remark that good practice is to introduce the symbol the first time it's used, as in: ... the gain margin (\$GM\$) of the circuit is 27dB.

So I'm mourning the loss of a perfectly good opportunity to use a fancy Greek letter in my technical writing, but I suppose I'll have to get over it :)