You can reset your GMail password using the web, but I think you'll still need to know the previous password to get into the phone.
If you cannot get your password the only way to unlock it is a factory reset, which will erase all data on the phone. This is a security measure: as you can appreciate, if there was a way of unlocking the phone without the password then any thief could use that to get access to your accounts and personal information.
How you would do a reset depends on the phone. The basic principle is to shut it down, boot into recovery mode and reset it, but how you access recovery mode depends on the manufacturer (it's usually pressing a combination of buttons while booting, but different manufacturers use different combinations). If you post again telling us what phone you have someone will be able to help - a web search can usually turn up the method as well.