For starters Michael Gilday is in Italy (or at least was 20 hours ago), whereas this document shows all the chiefs signing it on one document with the same pen. https://www.instagram.com/p/CJ7YN10l6r9/

Trying to figure out where other chiefs are now... These guys don't do social media much

Secondly, no official notice has been released by any government agency, including https://www.JCS.mil None of the .com sites sharing the same pdf will list their source.

edit: Thanks to u/confusticateBebother we can see Navy.mil does have a listing of the letter just from Gilday no signatures. This doesn't look good but then again the secretary of state site was just hacked yesterday and said Trump's term had ended and that ended up being a hoax... I don't know what to think about any of this, going to take a break from the screen for a bit and see if new news pops up when I'm back in a few hours. God speed, wwg1wga