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Pro Best support for Gmail features
Since both the Gmail Android app and Gmail itself is made by Google, the app supports all features Gmail offers (like labeling/starring) and gets new features implemented early - usually as soon as they are announced.
Pro Incredibly fast, advanced & predictive search
Gmail brings much of the functionality and power of Google's search engine to the app's search. You can use operators such as label:, is:, OR, has: among others, define date ranges, limit search to specific groups and so on. And, you can do it all amazingly fast. The search will also try to predict what you're trying to search for before you've even finished your query and offer up those results.
Pro Comes bundled with Android for free
Not only is the application free, it comes bundled with the Android OS by default so you don't even have to bother downloading it.
Pro Google Drive integration
The app allows inserting files from Drive straight to email or uploading attachments to Drive before sending them out. It will also notify if the person you're sending the file can open the file our should you change the sharing settings for that file beforehand. Finally, you can choose which one of your Google accounts you want visible in the app.
Pro Google Calendar integration
Pro Safely automatically displays images
The app does not require a confirmation to show images as Google now hosts the images on their own servers making it safe. You can still disable the automatic displaying of images in the settings if you wish.
Pro Tabs to automatically organize your e-mail
Gmail offers the ability to automatically organize your mail into specific categories:
Each of the categories can be disabled and you can move your mail around from category to category or set your own rules which sender should be put in which category.
Pro Threaded conversations
Con Can't sort mail by Unread
Nearly every other mail app has this ability. If you have more than one Gmail/email account or get a lot of email, this is a significant omission. *You can sort other, non-Gmail accounts linked to the Gmail app, but not Gmail accounts.