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Well there doesn't seem to be a select all button on the app. But on the gmail site you can select all (it maybe only that first page tho don't remember).
But an idea you could try is creating a filter with all fields blank and see if it returns everything. Then usually in the bottom right of the filter search is a little thing like "15 messages found matching" and it let's you apply your filter to the existing messages. If it returns all your emails you could apply a filter that deletes or archives them all.
Currently on my phone but ill update later with results if I get a chance to test my idea.
EDIT UPDATE::
So an easy way is do like I said set up a filter and search for email that "Doesn't have:" 'somebull****crazywordthatwillnotbeinanyemail'. It will return all your emails and then you can do whatever you want with them.