The wife’s grandmother is a mahram to the husband

The wife’s grandmother – either paternal or maternal – becomes a mahram to the husband once the marriage contract is done, because she is included in the verse in which Allah says (interpretation of the meaning):

“your wives’ mothers” [al-Nisa’ 4:23]. 

Thus the mothers, grandmothers and great-grandmothers of the wife, no matter how far the line of ascent reaches, are mahrams to the husband. 

It says in Zaad al-Mustaqni‘: His wife’s mother and her grandmothers become mahrams once the marriage contract is done.

See al-Sharh al-Mumti‘, 12/118 

It says in al-Mawsoo‘ah al-Fiqhiyyah al-Kuwaitiyyah (37/ 368): 

The fuqaha’ are unanimously agreed that it is permanently haraam to marry, due to marriage…the asl of the woman and they are her mother, her mother's mother and her father's mother (no matter how far the line of ascent reaches) and that is because Allah says (interpretation of the meaning): “your wives’ mothers” which is mentioned after “Forbidden to you (for marriage) are: your mothers” [al-Nisa’ 4:23]. End quote. 

Based on that: the grandmother of the wife is from your maharim and she may uncover her face in front of you and you may be alone with her like all other maharim. 

And Allah knows best.