If you spent both {G} and {W} to cast Dawnglow Infusion, you’ll gain (X + X) life. An ability that triggers whenever you gain life will trigger just once.

Whether you choose to spend both {G} and {W} to cast Dawnglow Infusion has no bearing on the value of X. For example, say you decide that X is 4 and you’ll spend {G} on {G/W}. If you end up spending {W}{W}{R}{R}{G} to pay the cost, X is still 4 and you’ve spent both {G} and {W}. You’ll gain a total of 8 life.

This spell checks on resolution to see if any mana of the appropriate colors were spent to pay its cost. It doesn’t matter how much mana of that color was spent; the effect isn’t multiplied.

This spell cares about what mana was spent to pay its total cost, not just what mana was spent to pay the hybrid mana symbol in its cost.

If this spell is copied, the copy will not have had any colors of mana paid for it, no matter what colors were spent on the original spell.