You can link your @spacegeneration.org account to your SGAC Web Account later on by going into “Your Account” > “Social Connect” and “Connect to Google+” and then signing into your @spacegeneration.org account there. After that, you can log into your SGAC Web Account by simply clicking the red “Sign in with Google+” button. If you have already log in to your @spacegeneration.org Gmail, you will be taken directly through login without needing to enter your details again.
You can read more about how this work here.