The same difference as with the other Silver version: steel and handle material. G4059 is 7Cr17MoV stainless with a silver aluminum handle. G1068144 is black-oxide-coated D2 tool steel with a copper handle. Both share the 4.75 inch closed frame and 3.5 inch blade.
7Cr17MoV in the Silver: high chromium, very corrosion resistant, modest edge holding, easy to sharpen on anything. D2 in the Copper: semi-stainless, notably better edge retention, wants a wipe after wet work and diamond or ceramic abrasives.
If edge retention matters to you, the Copper is the meaningful upgrade.
Silver aluminum versus copper with a black stainless back handle. Copper is denser, so the Copper version feels heavier in the pocket, and it will patina with handling.
Both carry a pocket clip, a lanyard hole and Gerber's pinch plate.
G4056 and G4059 are the same knife by specification: identical steel, dimensions, finish and handle. If you are choosing between those two, you are choosing between listings rather than between knives.
See the G4056 comparison.
Specifications above come from the manufacturer. Live prices and availability are in the strip below, pulled from the catalog at build time rather than typed into the copy, so they cannot go stale on this page.