Couple of questions first:
- Are you using a desktop client?
- Are you using Gmail's internal multiple accounts system?
If you're using a desktop client, you'll need to make a second outgoing (smtp) account (not the best word, but it describes it). I'm not sure how you would do that (I haven't used a desktop client for a while, but the google help page is where I got my instructions for doing it from.
If you're using Gmail's internal multiple accounts system you just go to gear -> mail settings -> accounts and import and in the send mail as section, choose "Reply from the same address the message was sent to".
If you've got that setup, and your problem is that you just can't send it from the apps address through Gmail without using multiple accounts/signing in/out, in the compose window, there's a dropdown box for the FROM field with all of the accounts you've setup to send mail as.