You can train Gmail to what you regard as junk mail (ie spam) by flagging messages as such. To do this, open a message in your Gmail app and open the menu by tapping the three-dots icon in the top-right corner of the screen. Here you will find two options that relate to flagging unwanted and suspicious messages.
‘Report as spam’ alerts Google that the message is junk and was not written specifically for you. The other option, ‘Report as phishing’ is for flagging more sinister messages that have been written specifically to trick you into giving up personal information.