Can Legendary Six Samurai Shi En destroy Fusion Gate when is played but its effect not used yet in Yu-Gi-Oh?

No, Legendary Six Samurai Shi En cannot destroy Fusion Gate when it is played but its effect has not been used yet. Here's why:

* Shi En's effect targets a monster: Shi En's effect states "Destroy 1 monster on the field." Fusion Gate is a Spell Card, not a monster.

* Shi En's effect is an activated effect: It specifically says "You can activate this effect..." This means the effect is not triggered automatically, but must be activated by the player.

* Fusion Gate's effect is not a monster effect: Fusion Gate's effect is a Spell Card effect, not a monster effect. Shi En's effect only targets monster effects.

Therefore, Shi En cannot target Fusion Gate with its effect.

Even if Fusion Gate's effect was used, Shi En could not destroy it. Shi En's effect only targets *monsters*, and Fusion Gate is a Spell Card, even after its effect is used.