This is one of their new mattresses. Cooling Essential Foam Mattress. I have never tried this one but we have the original Green Tea mattresses working for us for 7 years now and they have been great.
Might be worth a shot for $299. Reviews state that it is a pretty firm mattress. If you want to soften it up, drop a 2" or 3" memory foam topper on it. That makes the sleeping surface softer while still retaining the firm base.
That is what we have done.
EDIT - These kinds of flash sales often just end so if you need a king mattress, better jump.
Interesting . . . . Amazon says the firmness is "Medium" which is the same stated firmness as the Zinus Cooling Gel, yet at half the price I might have to jump on it. Thank you Mike!