It runs in the notification area and uses very little system resources as well. Once installed, you can enter your Gmail account information to the app and it'll fetch new Gmails for ya!. It'll open new mails via the web browser that's "set" by Gnome which also applies to the email client as well.
So if you want a different web browser to open your Gmail with cGmail, then set it via the Gnome preferred apps menu. Anyhow, if you use Ubuntu 10.10 or the 11.04 Natty Narwhal then you can install it by issuing the below command in your Terminal.
sudo apt-get install cgmailIf the command does not work in Ubuntu 10.10, then simply download the .deb package from here and double click on it after the download is completed.