Light gray grout will look very nice. I can't imagine that any sale decision will turn on a simple stylistic choice like grout color. If someone takes unreasonable exception to the grout, they can always buy a jar of that grout pigment/paint and spend a happy hour covering it up.

We have off-white cabinets and off-white subway tile (with darker soapstone counters) and chose a grout that is just one shade darker than the tiles ("alabaster"). It is entirely a personal preference, but I don't like too much contrast with the grout highlighting of the shape of the subway tile. It ends up looking busier than I like. So the subtle effect of a just slightly darker grout seemed to do the trick. In your case, with gray counters, I would make the gray grout very, very pale--just a hint of silvery-gray.

This is off-topic but I am about to do my kitchen Reno and have chosen white cabinets and backsplash and London grey quartz countertops. My husband thinks this will look too washed out and pale. Would you be willing to post some photos of your kitchen to help us visualize this combination? For what it's worth, I would go with a light grey car out (I know someone who did this in a white kitchen and it makes the tiles pop and gives texture to an overall plain palette. Good luck!

Sorry, that should have read light grey grout! (Darn autocorrect!)

Unfortunately, we are just in planning & design phase. Our kitchen has oak cabinets and white tile counters still.

There are some great pics on houzz which I have saved in my idea book. Hope this helps. Btw. We saw Frosty Carrina and fell in love. We are thinking this with white subway and white grout. (Our kitchen does not have much light and thinking the London Grey and grey grout will be too dull). Good luck.

my personal preference would be white grout

Here's an ideabook on this subject. You can make a statement or do something more subtle and still have a little contrast.
https://www.houzz.com/magazine/subway-tile-picks-up-gray-grout-stsetivw-vs~1613366

You are smart putting in the subway tiles

grey grout above makes it too busy.

Agree that the grey above is too dark, but a very light grey can be nicer than stark white.