Dear Student,
In solid and liquid the atoms, ions, or molecules are very close together. There is no space between the individual particles, so they cannot be pack together.But in gaseous state,the intermolecular spaces between the particles in gases are maximum and the intermolecular forces are minimum. The gas particles are thus very far apart and can be brought closer by applying high pressure. Therefore gases are highly compressible or have maximum compressibility .
Regards