In the release notes of version 184.108.40.206 you will find in Resolved issues:
When filtering HTTP2 traffic was enabled on Web Gateway, processing failed for gmail POST requests that had been sent using the Chrome and Firefox browsers. (1188899)
We use version 220.127.116.11 but we can't find a way to turn Http/2 on or off.
I am not sure about 7.6.x, don't have that running at the moment. But in 7.7.2 you can use the Enable proxy control event to override use (and max. header length) of HTTP/2. Since you're able to use any criteria in a rule where you use that event, you can do it just for problematic sites, combination of site/user-agent or globally.
maybe it is not wrong to consider upgrading to latest version 18.104.22.168 since there are several HTTP2 bugs/issues already fixed in 7.7.2.x.
Here you will find a summary of the release notes:
Since I am not sure if this article is always kept up to date, you can also have a look here:
I know how to configure HTTP/2 under 7.7. Unfortunately, we will have to work under 7.6 for at least another 6 months.
So the question how can I enable HTTP/2 under 7.6? In the Release Notes, several bug fixes are used to indicate the presence of the option.
This is just wrong information in the release notes. There is no support of HTTP2 in 7.6.
I also checked the bug ID and there is only information for 7.7.1 and 7.7.2. I will check this with the documentation team and get this PDF updated.