How much gravity in space station?

Gravity falls with the square of the distance from Earth's centre, not from its surface. With a mean radius of about 6,371 km, the planet's centre is only some 6% farther away from an orbit 400 km up than it is from the launch pad, so the local pull is still about 8.7 m/s², close to 90% of the 9.80 m/s² at sea level. An 80 kg crew member would register roughly 72 kg on a scale bolted to a fixed tower at that height. What is missing aboard the station is not gravity but any surface pushing back.

The station and everything inside it fall around the planet at the same rate, so the useful figure for science is not the gravitational field but the residual acceleration left over: typically on the order of one millionth of g. It comes from crew movement, pumps and fans, thruster firings, the faint aerodynamic drag of the residual atmosphere, and the gravity gradient that very slightly separates objects stowed far from the centre of mass. Sensitive experiments sit in racks with active vibration isolation to keep that residue as low as possible.

Drag makes the orbit decay, so the station has to be pushed back up periodically; during those reboost burns the acceleration on board climbs far above microgravity levels and delicate experiments are paused. Placement matters too, since the quietest volume is close to the centre of mass. Spinning the structure to create artificial gravity has been studied for decades but never flown on the ISS: the whole point of the laboratory is the absence of weight, not its recovery.