Professional linguist would be an accurate term for such a person, though it's not an all-encompassing one (because there are many 'grammar gurus' who do not make a living from their language expertise). We also have the terms English teacher and English professor for people who are (in principle) expert in English and disseminate their knowledge to others.
Terms like grammar Nazi and grammar police exist in response to the fact that:
1) not everyone welcomes having their linguistic practices commented on or 'corrected';
2) many self-styled experts cannot tell the difference between their own personal preferences and usages they are merely unfamiliar with, but apply their own prejudices regardless; and
3) some of them have a habit of claiming greater knowledge or expertise than they actually possess.
Incidentally, it is unlikely that I would describe myself as a grammar queen even if I possessed the necessary level of expertise to qualify for the title.
Regarding the term grammar guru, I have the following observations:
1) 'Grammar guru' does not occur in the OED as an established term.
2) The citations contained in the OED's entry for guru (of which a clone can be found here) refer solely to the term's religious usage until 1957, after which it occurs in a secular context, sometimes as a less-than-complimentary term.
(For instance, an article in the London Times dated 4 October 1968 reports the British Prime Minister Harold Wilson talking about Enoch Powell, the Conservative MP for Wolverhampton:
"In what was obviously intended to be a sneering reference, the Prime Minister in his facile glibness..described Mr. Enoch Powell as ‘the guru of Wolverhampton’."
Here, Wilson was using 'guru' as a snarky epithet for a political opponent whom he regarded as a false prophet and troublemaker — in April 1968, Powell had delivered his notorious 'Rivers of Blood' speech opposing new legislation that would make it easier for immigrants from the Commonwealth to settle in Britain.)
My attempts to pin down the earliest attested occurrence of grammar guru have been largely fruitless.
The term is not found either in Brigham Young University's Corpus of Contemporary American English (COCA), its Corpus of Historical American English (COHA), its Strathy Corpus (of Canadian English), or its British National Corpus (BNC); nor in Oxford University's BNC. I have searched other corpora too, but with the exception of BYU's Corpus of Global Web-Based English (GloWbE), which contained only a handful of what appeared to be relatively recent hits, these too were devoid of the term.
For what it's worth, the domain name grammarguru.com was (according to Whois.net) first registered in 1999.
The nearest I could come is a 1997 reference to slang guru by Jonathon Green, the renowned lexicographer of English slang, describing Eric Partridge (the author of inter alia A Dictionary of Slang and Unconventional English).