Alternative Gravity Theories
Non-mainstream gravity theories tend to fall into one of three different types..
The 'expanding universe' is based on the idea that after the initial Big Bang, or equivalent, all matter in the universe started flying apart due to an inherent energy pressure. Everything looks the same to us because the atoms that we are made up of are expanding at the same rate as everything else. It is this accelerating expansion that gives us our 1g force here on earth.
The 'shadow effects' are quite popular. The concept assumes some undetectable and usually massless particle similar to photons that permeate space and are travelling chaotically in all directions. These virtual or massless particles only exert a net force on matter if they are partially obscured by another lump of matter. It is the shielding that each body creates that causes an imbalance in the overall force and the two bodies will be mechanically pushed together.
Nicolas Fatio de Duillier and Le Sage (circa 1690-1748)
Gravity wave theories are more mainstream and the Ligo experiment is an attempt to check for the existence of these waves. In February 2016 physicists from the Ligo experiment annnounced the detection of gravitational waves which are described as ripples in spacetime. To make a ripple in spacetime is not at all difficult, just wave a lead weight about and you will modify spacetime enough to be able to detect it.
Extremely low frequency waves with very, very long wavelengths are the same as gravitational waves, see https://en.wikipedia.org/wiki/Extremely_low_frequency. They have the same ability to penetrate relatively easily through matter, sea water for instance, and are used to communicate with submarines. All EMF waves are ripples in spacetime. Gravitational waves do not cause gravity, it is the "current" that causes gravity not the "wave".
Gabriel LaFreniere (archived website -click on the 'impatient?' button)
Caroline Thompson's Physics see specifically The Phi-Wave Aether: a Wave Theory of Everything
also, Wikipedia has some interesting reading on the mechanical explanations of gravitation