Most PCs running with 8Gb of RAM will start to struggle with a few browser windows open, multiple tabs and a few other apps running. I suggest you upgrade to 16Gb RAM. A site called crucial will tell you if you can and what chips you need and sell them to you.
As the other reply says, Gmail is accessed from a browser window, just go to Google.com and click the Gmail button and sign in with your user name and password.