Yes, when you integrate your primary Gmail account, SmartReach will automatically pull in all the Gmail Aliases as well.
You can send email campaigns from a Gmail Alias, but you can not receive replies to the Alias address.
To receive replies to those campaigns, you can choose your primary gmail address or any other email account that you may want to receive replies to.
Each gmail alias that you send from is considered a different email account, so to send from e.g. 2 aliases, you need to pay for 2 x subscription price.