Syr Gwyn's triggered ability triggers once per attacking creature that's equipped, regardless of how many Equipment those creatures each carry.

Equipment you control still have their other equip abilities in addition to equip Knight {0}.

Whether the target creature is a Knight is checked only as the equip Knight ability is activated and as that ability resolves. If the creature somehow becomes a non-Knight creature later, the Equipment remains attached to it.

If Syr Gwyn leaves the battlefield after an equip Knight ability has been activated, that ability still resolves even though the Equipment has lost that ability.