my theory is the gman in halflife is a multidimensional advanced lifeform. his employers are other multidimensional lifeforms. their interest is monitoring single-dimensional lifeforms as they grasp dimensional and temporal travel. their interest in earth is because of dimensional travel experiments and because of the dimensional rift attracting the most powerful single-dimensional empire that can dimensional travel, an actual threat. their interest is in controlling the proliferation of intradimensional teleportation.