Gutter Guardian Can I showoff Gutter Guardian itself when it's at hand as requirement to get the showoff? Since the text said as you play me, so as I play Gutter Guardian I showoff hidden card which is Gutter Guardian itself?

No — Gutter Guardian can't show off itself. During finalization it's a chain item, not a card in your hand and not yet a game object on the board, so there's nothing to reveal or pick (Rule 832.1.c).

If you're playing it from hand normally, you'd need to reveal a different card with Hidden from your hand (a second Gutter Guardian works) or pick another friendly face-up card with Hidden already on the board — its reminder text says "pick another friendly one." A facedown hidden card doesn't help either, since a facedown card isn't "a card with Hidden" (Rule 811.5.a).