The tag description for "grammaticality" says, "Grammaticality refers to whether something obeys the rules of grammar for English." The tag "grammatically" has no description, but its 23 questions all seem to be concerned with whether something obeys the rules of English grammar. Four of the questions under this tag follow the format of "Is ________ correct/grammatically correct?" and several others ask which of two options is correct. It seems that all of these questions fit the description of the "grammaticality" tag, so does the "grammatically" tag have some other purpose that sets it apart?
I have suggested grammatically as a synonym of grammaticality. I believe that if the synonym is approved, re-tagging becomes unnecessary, but that may just be wishful thinking on my part. In any case, if the system will let you, please vote on the synonym suggestion.
(It should go without saying that I believe the "grammatically" tag is a typo for "grammaticality".)
ETA: the synonym is now approved (thanks for the last vote, oerkelens!), and I was right: the system automatically re-tagged all the relevant questions.