I don't have a Pixel, but I'm having the same issue on my N6. And I've had it periodically on a number of devices. For me, it's typically email from early morning hours. I thought it might be Doze, but I have Gmail ignoring battery optimization. Ironically, I never have this issue on my iPhone, which, I just discovered, will push Gmail with the Outlook app.
I'm having an issue with Gmail on my Pixel XL as well. I will send a message from my phone and it will just say "Queued". And doesn't deliver. Sometimes for awhile.
If it happens early in the morning that could be doze. happens to me to with my devices. I expect that as my phone has been in a super deep sleep.
Well it still doesn't make sense. I've used my phone all morning and afternoon, an email alert just came through at 2:29 CT for something that actually arrived 12:16 CT....
This is unbelievable for a $1,000 device.
So your sure its the device and not the service?
theres a good possibility its a server side issue. Especially if its happening on other devices.
I have noticed a difference in sync between my Samsung S7 Edge, HTC 10 and Nexus 6.
Heck ill even notice a difference if they are all connected to the same wifi at home.
As i mentioned before its probably a server side issue.
I have a HTC 10, Note 4, Galaxy S6, HTC M9 and Galaxy S7 Edge.
I have noticed huge differences in gmail sync across them all. I dont believe its a device issue your seeing but rather a sync / server issue.
I have a couple gmail accounts and when i go into accounts and check sync some have not synced for several hours while others are a minute or two behind.
Try to get your hands on another device and test it out on that as well like i have and see what your results are.