Hi, Yuniati!
I have to give you what my kids call a "but" answer. It's where I give an answer, but then I tack on the bad news part at the end.
Walt Disney World does still sell
theme park tickets at the gate.
BUT, even if you purchase a ticket at the gate, you still have to have a
theme park reservation. I double-checked the
theme park availability calendar, and as you already know, there are no reservations available that day at
Magic Kingdom. I want to first recommend that you keep checking. People do change their plans, and you may find that an available reservation opens up. Keep refreshing that calendar.
Another option, which is not a guarantee by any means, is a
park hopper ticket. With this ticket, you can make a reservation at one of the theme parks that is available, You would have to go to that park first to scan in. Then, after 2 PM, you are free to park hop to one of the other theme parks. A HUGE word of caution, Yuniati. Although it doesn't happen often, there is a chance that a theme park may not be available for park hopping. So you could purchase your ticket, book a reservation at another park for the morning, then not be able to hop over to Magic Kingdom. It doesn't happen often, but it may. I'd suggest this option only if you would be okay not being able to hop to Magic Kingdom.
I wish I had a better answer than a "but" answer, Yuniati. I'll keep my fingers crossed something good happens in the next few days!
Anna