I know this is a late response, but I had just tried going to the gifting page, https://www.chess.com/membership/gift, and see what you mean with the greyed out buttons.
Once I filled in the username of the member I would like to gift the premium to, the radio buttons under the Gold / Platinum options became clickable. While the buttons for Gold and Platinum aren't as vibrant as the Diamond option (probably to visually entice purchase of this plan), it does allow me to select Gold or Platinum.
Hopefully this helps someone else out! It's also possible it was a bug or something 2 years ago when you originally posted this and they've since fixed it.
Did they remove possibility of giving gift of gold/platinum membership?
When i try to buy membership for my father, it allows me to choose only diamond option. Option for gold//plat are greyed out.
Thanks