As far as I know, after having a look around, the way to get to the page where you may use them is to go to your My Gaia tab and click it, then
Go to the cog icon at the far right and click the drop down box to Gaia Cash Purchases.
On that page, under the text that says "Your current Gaia Cash balance is: " you'll find a few links in blue text, one of those is to "redeem a cash card"
Clicking that should lead you to a page where you may redeem your card. I do not know if this page is ~meant~ to be used, since the cards are not being made available anymore, but it must still work if Gaia have provided a link to it still.
I hope that works for you, I haven't used this means myself, I just found how to find that page via another thread.
If it doesn't work, I suggest you file a billing inquiry via Contact Us and ask if your card can still be honoured.
EDIT: Whoops. Totally misunderstood what you wrote. There used to be a link on the GCash page, but since they changed the tabs around a couple months ago it's been gone, I think. You can go here still, I think, to use cards.