You just add each different account to the phone. I have 2 - my default account syncs gmail, calendar, and contact and my 2nd account syncs gmail only.
When you click the gmail app, hit the 'Menu' button and select 'Accounts' - you can toggle between the inboxes. Then you just send from there.
If you have your default account pulling mail into its inbox from the other accounts (one of the settings on the web), then I'd suggest you turn this off. When I got my BB Curve 8330 I initially had my default gmail account pulling mail from my second account - kind of like a "unified" inbox when I was on a PC and went to Gmail. But, I was getting duplicate emails from the 2nd account because it would download the message from m default account and from my 2nd account.
If you want a "unified" inbox where you can choose the "from" address to use, I don't think the Gmail app works that way. There may be a 3rd party app (or the HTC Mail app) that does this, but I don't use them so I don't what's available.