Shred's effect is referring to anything that would try to alter Shred's damage. Neither Aspertia City Gym nor Giant Cape affect Shred's damage. They would take 90 damage as usual.
Eviolite, however, is an example of a card that does try to alter Shred's damage. That's what the attack ignores, so you would still do 90 to a Pokemon with Eviolite attached.