Gray sport coats are notoriously difficult to wear. In most fabrics, they look too similar to orphaned suit jackets, which can leave you looking like you got dressed in the dark.
The big exception is the gray tweed, especially in a chunky herringbone or speckled Donegal wool. It goes naturally well with denim, which is nice in a day and age when men are trying to find more ways to dress down their tailored jackets. Wear the combo with casual shirts, such as oxford-cloth button-downs or chambrays, then lose the tie for a more comfortable look.
You can also pair one with moleskin trousers. Although a slight step up in formality from tweed-on-denim, brushed cotton moleskins will still look solidly casual. Again, wear them with causal shirts and maybe a wool tie (if you want to look dressier), or turn to a good, wool turtleneck, like our friend Greg in NYC above.
A gray-on-gray combo is a bit tricker, but doable if you have sufficient contrast between your jacket and pants. Voxsartoria above shows how you can make the combination a bit more colorful with a navy tie and brown suede shoes. Conversely, you can go full monochrome by pairing the two with a white button-down shirt, black knitted tie, and black pebbled-grained wingtips (or try a charcoal turtleneck for something a little more chic). The upside: a gray tonal combination looks a lot more sophisticated at night than things with too much color.
Of course, brown and navy sport coats will always be easier to wear. With the exception of jeans, the above combinations require more thought to put together (as well as probably a bigger wardrobe). If you already have a few sport coats in the closet, however, and you’re looking to add another, a gray tweed isn’t a bad bet.