Can all the forces be unified with gravity?

All the forces come from one structure, a network of twelve-port patches repairing disagreements with their neighbors, though no single grand symmetry holds them. The three forces of particle physics are a patch's twelve reversible responses, split as one plus three plus eight. In 1974 Howard Georgi and Sheldon Glashow proposed one larger symmetry containing all three. It has no room for the one dial that commutes with everything, and the twelve include no direction coupling color to weak charge, so the force-carrier route to proton decay those theories predict is absent. Fifty thousand tons of water under a Japanese mountain have watched since 1996 and seen none. Gravity comes out of the same network by another road, as the equation of state of settled regions, and it picks out no forces of its own.

Also in Chapter 33: Why Was Every Open Problem the Same Problem?, Chapter 25: Why Does Gravity Look Like Geometry?.