The Golden Tree is a proven easter egg in GTA V.
It can be found on the east coast of San Andreas, near Braddock Pass on Mount Gordo. The tree appears to look normal in the daytime, but at night, the tree's texture changes to a bright yellow color, giving it the appearance that it is glowing. The tree is easily noticeable from the nearby highway.
The tree is an easter egg, and players have even theorized that it is related to the Mount Chiliad Mystery. The tree is never mentioned in the game's storyline, and its nature is never explained.
There is a possibility that the golden tree may be a glitch. In areas of the Grand Senora desert there are in fact bushes that glow gold at night; leaving the possibility that these phenomenons are simply texture or light glitches that only occur at night.