No. There is a difference between 'activating a spell card' (ie, placing a spell card face-up in the S/T zone or flipping it face up from being Set) and 'activating the effect of a spell card' (ie, the reusable effects on a continuous card that activate and use the chain).
Magical Citadel of Endymion only looks for the former. It will only gain the spell counter from placing Gateway of the Six face-up on the field, or flipping it from being Set. It will not gain spell counters when you activate any of an already-face up Gateway of the Six's effects.
No magical citidel of endymion will only get a spell counter if the opponent plays a spell card. The effect of gateway of the six is being used and therefore since it was already played the endymion will not get a counter
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No. There is a difference between 'activating a spell card' (ie, placing a spell card face-up in the S/T zone or flipping it face up from being Set) and 'activating the effect of a spell card' (ie, the reusable effects on a continuous card that activate and use the chain).
Magical Citadel of Endymion only looks for the former. It will only gain the spell counter from placing Gateway of the Six face-up on the field, or flipping it from being Set. It will not gain spell counters when you activate any of an already-face up Gateway of the Six's effects.
No magical citidel of endymion will only get a spell counter if the opponent plays a spell card. The effect of gateway of the six is being used and therefore since it was already played the endymion will not get a counter