This is for Outlook 2003. Other versions should be similar.
Click Tools, point to Send/Receive, point to Send/Receive Settings, click Define Send/Receive Groups... (or just press CTRL+ALT+S, or Tools | Options Mail Setup tab Send/Receive... button)
I don't remember if there is a default group or not. If so it likely has all your accounts in it. If you use Outlook to download from your ISP account, you'll need 1 Group for your ISP account (and any others) and one for the Gmail account.
Create a group named Gmail and add your Gmail account to the new group. Uncheck all of the boxes for including the Gmail group in Send/Receive, or for doing automatic Send/Receives.
Edit all other groups and remove the Gmail account from the other groups.
You won't be able to use F9 to send, but you can add a Send All button to the tool bar.
Another option, if you do use Outlook for your ISP account, is to set it to use Gmail to send, or create another account with Gmail to send, your ISP to receive, if you need to be able to send from your ISP account on occasion.
HTH
Jerry