The colors you choose to complement your floors are very much reflective of what sort of style you want. If you are looking to create a modernistic and simplistic look, then you could create a great combination with black and white.
You could also splash out some more vibrant colors to give the décor a stronger sense of vibrancy. A coat of striking red paint on the surrounding walls would give the rooms a strong appearance.
Likewise, you could use a collection of blues to really complement the gray color. Sky blues and other pastel colors would create a comfortable and well-mixed appearance, but you could always step it up a level and use bolder blues and navy blues.
Deviating away from the primary and monochrome colors, you could get experimental with some more abstract tones and shades. Burgundy or green furniture and décor would give the room a beautiful blend of colors.
You could also get a mauve color; which resides in the color range of lilac or lavender. This would complement the gray colors greatly and give the entire room a great blend of tones and shades.
As stated earlier, gray is quite neutral and should really go with most other colors. However, it is down to your own personal discretion as to what colors you use alongside it. Once you have decided what sort of vibe you want the room to have, you can begin choosing the ideal colors for the job.