Gold Experience Requiem

Gold Experience Requiem (ゴールド・エクスペリエンス・レクイエム) is a stand of Giorno Giovanna featured in Part V: Golden Wind. It is the evolved form of Gold Experience created by piercing its body with the stand-creating Arrow.

Appearance
When Gold Experience Requiem first appears, it wears the Arrow on its forehead, but eventually the arrow falls to the ground. It is unknown if Giorno keeps Requiem's form after the arrow falls to the ground.

Silver Chariots loses its arrow but remained in its requiem form, possibly due to Polnareff's physical death. It's very likely that Gold Experience retains its Requiem form after defeating Diavolo, as Giorno is still alive.

In the official novel "Purple Haze Feedback", Giorno uses his Gold Experience Requiem to heal Fugo without Fugo noticing it.

Abilities
Nullification of Cause and Effect: Gold Experience Requiem's ability is to turn anything, including his opponent's attack and willpower, back to the state of "zero". Hence, it is near invincible, as all "supposed" actions created by an opponent would have been reset (back to point zero). It is mentioned by Giorno himself that he is unsure of the exact workings and limits of Requiem's powers, but it has been shown that its range of influence extends from finger beams, surface contact, and to even being in the mere attention of Requiem.

Requiem's abilities also seem to be opposite King Crimson's: King Crimson erases the "cause" and keeps the "effect" (i.e time where a bullet would hit him is erased, causing the bullet to warp past him), while Requiem keeps the "cause" but erases the "effect", causing endless scenarios where actions and initiatives are started, but the effects of such does not occur (as shown when Requiem completely halted King Crimson's cause erasing ability).

If the opponent is "killed" by the stand itself, then they will also continuously experience death, as they will die but repeatedly return to point "zero" (i.e Diavolo's endless death loops).

This ability renders Giorno essentially untouchable by anything that would try to hurt him and is perhaps the strongest defense in the series.

Original Abilities: It is often disputed whether Giorno retained his original life giving abilities after evolving into Gold Experience Requiem. After fusing with the arrow, Giorno breaks a column and a piece of it turns into a scorpion that then attacks Diavolo, so it is very likely he still has all of his original stand's abilities.

All Star Battle
(The information below derives from videogame. As such, it may or may not be considered canon.)

In All-Star Battle, it was shown that Gold Experience Requiem functions normally under the time-accelerating effect of Made in Heaven, and Giorno remains immune to the effect as long as he has Requiem active, while other characters would be severely slowed down by it.

Trivia

 * While whether the events of All-Star Battle can be considered canon or not is unclear, the fact that GER can negate the effect of Made in Heaven lends some credibility to the fan theory that Giorno might've managed to accomplish the same feat in the actual manga, and as a result could exist somewhere in the alternate continuity that started with Steel Ball Run.