From Sonic Retro
The Gold Shield, known as the Gold Barrier (ゴールドバリア) in Japan, is a power-up exclusive to Sonic 3D: Flickies' Island, and a variant of the standard Shield.
In addition to serving as a one-shot barrier against damage and life loss, the Gold Shield allows Sonic to perform what the instruction manual called the "Sonic Blast Attack": a primitive version of the Homing Attack. Double-tapping or would cause Sonic to jump and home in on a Badnik (over a limited range). The shield couldn't be used to execute a Jump Dash, however; if there were no enemies in range, a double-jump would just bring Sonic crashing back to the floor, similar to the Stomp move of later games.
Other variants of the basic shield include the Lightning Shield, Fire Shield, and Water Shield.