Discounts on Memberships
Unfortunately at the moment Gumroad memberships do not have a "Temporary Discount" feature where members could, for example, get their first month free, then pay $5/month.
Any discounts applied to memberships are therefore permanent. Whatever rate someone starts paying on a membership product is what they will always pay for that membership.
What you can do is issue partial refunds on customer's subscription installments. Simply pull up the customer in your customers tab, pull up the individual installment, and enter the amount you'd like to refund: