What is an alias?
An alias is another email you can send and receive emails from.
ProsperWorks needs you to log in with your default email address. To figure out what is your default address, go to your Gmail -> Settings -> Accounts and it will show up in that view.
Here you can see Harrymoonshine1@gmail.com is my default and email@example.com is the alias.
This means I have to sign up with ProsperWorks using Harrymoonshine1@gmail.com, but I will still be able to see emails from firstname.lastname@example.org in my ProsperWorks account.
ProsperWorks will pull emails in from your default email and all the aliases you have. If you do not want to pull in emails from all aliases into your ProsperWorks account, you will have to delete them from your Gmail.
More instructions on adding aliases from Google: