As Fluffhead said, the Gmail application will not work because it opens a Chrome link for that link, which does not make sense.
Two options for accepting an internal tester invitation in a mail from iTunes:
1) Use iOS Mail to click the link in the mail, then iOS Mail will automatically open Safari.
2) If iOS Mail is not available, copy the link and paste it into Safari. For example, you can use Gmail to click a link, copy the link in Chrome, and paste it into Safari.
Neonberry
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