It is not clear from your email if you tried saving the attachment from the gmail email, and then closed your browser, launched Adobe, and then tried to open the newly saved .pdf file.
I have seen instances when you cannot just double-click the attachment and it opens in Adobe. My next step is always to take the web browser and possible file locks out of the equation.
Just a suggestion. Have a great day, and good luck.
Regards,
-Phil