Disclaimer: This is directly from Android Central. They have a *great* series of articles on getting started with your Android phone and getting the most out of your Android phone. I encourage you to check out their Android Tips and Android 101 series. I'm featuring this one because I've had to show several folks how to do this in the past couple of weeks.
In the most recent version of the gmail app, the options to reply all and forward e-mails are somewhat hidden. You've got top-level access to replay to the original sender, but to reply to everyone in the e-mail, or to forward the e-mail, you'll need to first hit the little triangle button beside the 'reply' icon. Then you'll have options to reply all or forward.