If you connect directly to the internet via a modem (i.e. you don't have multiple computers connected via a router), then the Default Gateway address will be allocated by your ISP along with the other addresses - IP Address and DNS Server Addresses.
If you open a Command Prompt window ("Start > Run", type CMD and click OK), type IPCONFIG /ALL and press Enter, the details of all adapters installed in the PC will be shown. You should see the Default Gateway address in there.
If most of the sites you are unable to access are secure "https" sites, the problem may be due to an incorrect MTU (Maximum transmission Unit) setting. Download CyberTweak from click here and try reducing the MTU setting to something like 1450 and see if that helps.