Can a Magic/Friend be used to damage a gargoyle which says "All damage done to this card is reduced to 0, with the exception of magic damage"?

No. Magic/Friends do not deal Magic damage, they are just played in the same manner as Magic cards.

To further clarify...

Magic/Friends are only Magic when they are in your deck, in your hand, or in the discard pile. Once they hit the field, they are considered a Friend only.