If your perception is that the seagulls are pointing straight into the wind but moving sideways to the wind, I suspect that you are mistaken. When the wind is strong and the groundspeed is very low, it only takes a slight difference between aircraft (or bird) heading and wind direction to produce a large crosswind component in the ground track. This is certainly true for gliders (and airplanes) as well as for birds.
So "sliding sideways" in relation to the wind is accomplished by steering the aircraft (or bird) to point slightly to the left or right of where the wind is coming from. In strong wind, an aircraft or bird can move perpendicular to the wind direction, with no "forward" progress, while still pointing almost directly into the wind with wings level. Note that this does not actually involve any sideways component to the airflow over the aircraft (or bird).
If the seagulls you observed are maintaining altitude while doing this, without flapping their wings, then they are undoubtedly in slope lift. Slope-soaring on a windy day often involves pointing almost straight into the wind, even while the ground track follows the line of the slope, which may be perpendicular to the wind.