It is important to note that your Momentum account and Google account are not the same. We have plans to add a Google account login option in the future. In the meantime, you'll have to create a separate Momentum account, using your email like a username.
When you download Momentum you are asked to create a unique Momentum account by providing an email address and choosing a password that you will use to login with. When creating any new account online we highly recommend that you choose a strong and unique password.