From what I see, it's way too huge to be anything like a dust "spot". It looks to me like something has gone wrong with that portion of the LCD panel in the EVF assembly (separating layers maybe). It's likely to get worse with time, I had the same thing happen to the LCD panel on a 42" TV.
Your options are to try to ignore it for as long as you can, or send it to an authorized Panasonic service center (since you're in Florida the center in McAllen TX would be best for you) for a service/repair estimate.
Third option (and this is what I would recommend) would be to shop for a used GH4 replacement. The EVF in that model is MUCH improved over the GH3. Watch the Buy/Sell sections here on this site and on mu-43.com. I would do that before investing in repairs on a GH3.