I tried to use the Pharmacy Rewards code at Target.com to make a Disney GC purchase and I got a warning that "the items in your cart do not meet the minimum requirements for the promotion you entered". This happens regardless of the amount of GCs that I put in my cart.
I added a regular item ($2 sunscreen) and it applied the discount to just it and not to the GCs, so I know that the code itself is valid.
So, from what I can see, the Pharmacy Rewards do not work online to get 5% off of the GCs.