Gmail now supports push mail to mobile devices
According to Official Google Mobile Blog, Mail sync function was already added to Google Sync service.
I have used Google Sync on my Nokia E63 to sync Google Calendar for a few month. I just added Email setting to Mail for Exchange and got push mail from Gmail.
Official Google Sync document does not mention about Email service yet (2009/Sep/24). Current online document just mentions about Google Calendar and contact list synchronization. I suppose you can easily add Email setting to this.
For Nokia S60 devices, you just need to have Mail for Exchange application. It is free application and you can obtain it from Ovi.