Gravity can be measured by producing a local acceleration which cancels it out.
Gravity is exactly detectable as a local acceleration and can be precisely measured.
The biggest problem with gravity is that it enters into the equations of motion for a particle as a part of space and time. This means that we have to find out the structure of space from the distribution of masses and then find the motion in that warped space. This is difficult. Another aspect of gravity is that it is very weak. The excitations of gravity, the gravitons are expected to exist, but are so weak that none can seem to measure them.