What connects gravity directly to the EM force and nuclear forces, is pressure. Gravity compacts matter to create pressure which pushes atoms togethers. This causes the EM forces between the atoms to see a potential increase. We can revers this by adding heat to a mass body, bound by EM forces. The matter expands, thereby lowering the mass/density and gravity. We altered gravity via EM.

Pressure is force/area. Since gravity is forming pressure, the force behind the pressure, is the gravitational force being applied over an area. This causes the EM force to change via the area interface.

The reason the simple path of pressure is not used to unify the forces, is because it appears to conflict with GR, which is assumed to fully define gravity. Gravitational pressure lowers the distance between matter, just like GR contracts distance in space-time. However, as we lower these distances, via gravitational pressure, the transitional speeds of matter gets faster. This means pressure is causing time expression of matter to move in a different direction than GR.

As an example, space-time in the center of the sun is the most distance contracted according to GR. This is the bottom of the space-time well. If you look at the matter in the core, it is packed closely, consistent with distance contraction of GR, but it shows gamma speed time transitions, which is very fast. It does not parallel the slowest time at the bottom of the space-time will. Space-time is going one way with respect to time; slows, but with respect to time transitions in matter, via other three forces, time speeds up; gamma.

One way to see this is we measure time with atomic clocks, which is based on the natural properties of atoms. The atomic clocks; nuclear atoms, in the core of the sun tick much faster than the atomic clocks n the surface; plasma atoms, even though GR says time and clocks are supposed to runs slower in the core. This is due to gravity and GR, being converted to the other forces as a result of force/area. All forces are unified in the sun via pressure.

If we had pure mass and no other forces, maybe GR would be all you need. For material transitions in the core to slow, to parallel time at the bottom of the space-time well, going from surface to the core would need to get colder; in the direction of absolute zero. This is not the rule in the universe, but rather matter will also have EM and nuke forces, is more the rule. The pressure interface hands gravity off to the other forces to make material time transitions; atomic clocks, get faster and faster. We are not talking about just gravity and GR, but how gravity hands off to the other forces via pressure for another layer of affects where atomic clocks tick faster.

The pressure interface between all the force, suggests unified force is mediated by area. It is not done in 1-D.