My 2 cents on this.
There may be several things going on, but ultimately there is a larger issue going on between Apple and Google than individual people's ISP's and their gmail forwarding settings.
This is about e-mails FROM Google's server TO Apple's servers.
Whether those e-mails are just auto forwards from an individuals personal gmail to their own personal iCloud or if people only use iCloud (@mac, @me, @icloud) and are just getting day to day mail from Gmail users to their accounts, some mail is getting delayed.
Apple is very aware of the problem. Yes some tech's are not. It all depends on how they initially log your description of the problem if their script associates it as "the" problem we are all talking about or one of the many other possible causes for delayed inbound e-mail
They are working with Google to sort it out. There is finger pointing going on from both sides. To defend Apple a little, unless this is truly about Apple vs Google politics, I have to imagine that there is a deeper issue, otherwise this would be solved by now.
At this point, all we as users can do is continue to call them to make our voices known. They do know, but it it never hurts. Changing our individual settings etc... wont' fix this problem. It is something they need to do.
Now there are other issues that can cause similar issues (compromised e-mail, x e-mails sent in y hours, mx block lists / black lists etc...) these can be both on the apple side, or the gmail side. But thats a whole other topic.