The cookie problem is generated when the app is restored using BlackBerry Link, or when BlackBerry OS is updated. Please delete the EmailForGoogleGmail app and re-install it from My World of BlackBerry World on your BB. (And No, you dont need to repurchase the app. You can download apps from My World of BlackBerry World as many times as you want)
To open an attachment, please long-press it, then select Open In Browser.This is due to API limitations.
This page explaines what to do step by step.
To select emails in the main screen, have a longer tap (than when you tap to open an email) on the small area close to the left corner of the screen (don't worry to touch the bezel outside the screen). Otherwise the email gets opened instead of getting selected.\nThe above is also valid when you Star an email.
This app is browser based. To avoid startup delays, try leaving the app running (thumbnailed). It does not run anything in the background, so it doesn't effect the battery life.