Due to the "footer" of a phab email alert not altering within a thread the link to the task is collapsed by gmail, requiring the user to first expand the collapsed region before being able to go to the task, if the link were moved up to the top of the message this would not happen.
I have been thinking about this as well. Then again, the current behavior saves you the common URL appearing in every post of a thread, and it is fair to think that users only click on a % of notification threads (the ones they plan to reply probably).
I had typed the task upstream and I was about to press "Submit", when I though that maybe the current behavior is actually better.
Upstream, https://secure.phabricator.com/D20561 will add a "View Task" button to the upper-right corner of HTML email from Maniphest.
The underlying issue is sort of a symptom of GMail's trimming behavior: GMail folds/trims identical suffixes of threaded messages. A "fix" for this behavior would be to choose a point in each email where subsequent footers are redundant and insert some random text there, so the mail would become:
TASK DETAIL https://phabricator.wikimedia.org/T1050 (Shhh, ignore this random text to stop GMail from folding above this line: alsdknfalkndf23infn1nferwnlk) EMAIL PREFERENCES https://phabricator.wikimedia.org/settings/email/
In HTML mail we could probably effectively hide this text (by coloring it white and/or making it 1px tall or just putting it in a display: none element).
I dislike adding Gmail-specific features to the upstream and I think the "View Task" button is probably good enough here and likely resolves the actual issue, but it's something to think about.