Using Gmail app on a public WiFi
The GMail application is now a general email application. You can set it up to talk to any email account. There are two important cases:


You use the GMail application with a GMail account, for example: my.user.name@gmail.com. In this case, Google only allows secure connections. 
You access a non-GMail account using the GMail application. This may be encrypted or unencrypted depending on the configuration of server and client. You can configure this to be unencrypted, though any decent mail provider now should only allow secure connections. 


Attackers will not be able to read your data unless they can generate a valid certificate that matches your mail server address or can otherwise compromise the TLS connection. This is not impossible, but if your server and your client are up to date on security patches, this is not something you have to worry about on public wifi.