does factory reset delete my gmail contacts, drive files, google plus photos e.t.c?
Any data that is synced to your google account is safe. Factory Reset will only wipe data that is stored locally on your phone. Resetting your password won't hurt, but it's probably not necessary, especially if you use multifactor authentication.

Update

Ars Technica is reporting on a study that shows some information on some devices may remain after a factory reset. This could potentially include login credentials. For extra protection, it is recommended that you encrypt your device with a very secure password (more than 11 characters) prior to doing a factory reset. This will make recovery of that data less likely. I would add that many services allow you to revoke access from individual devices. For instance, devices may be removed from Google by going to security.google.com/settings/security/activity, selecting your old device and clicking remove.