It isn't currently possible to merge separate Google Accounts. However, if you'd like to transfer your data from one account to another, this may be done on a per product basis. Or, to start using a new product, you don't have to create another Google Account. Sign in with your existing Google Account, and visit this list of products to get started.
If you don't have Gmail, you can add it to your account at any time.
If you signed up for Gmail and didn't add it to your existing account, now you have two separate accounts.
Note: if you're adding a new Gmail address to your Google Account, you can't choose an existing Gmail username (even if you delete the other account with Gmail, you can't reuse the Gmail username.)
Nick is an Accounts expert and the author of this help page. Leave him feedback below about how to improve it.