First thing is make sure that the install location is not in the Windows or Program Files folders. If it is you may get problems. Try to move it or reinstall to a drive root location e.g. (c:). Check that your set up as Administrator. Once installed right click on the GTL desktop icon and make sure that the Compatibility Mode box is un-ticked. You can tick the run as Administrator box. To get on-line for Multiplayer racing you need to fill in the Registration page of GTL which you will come across when you start Multiplayer GTL for the first time. Here you enter the On-line Key from Steam (CAPS for the letters, with NO hyphens (dashes) between them), your driver name, password and email address. Sometimes this takes several attempts to work so be patient. I have created this thread which shows the various steps in pictures
https://www.simracing.org.uk/smf/index.php?topic=13966.msg252396#msg252396To unlock all of the GTL content without doing the cup challenges you can follow the instructions here
https://www.simracing.org.uk/smf/index.php?topic=15379.msg280221#msg280221. Start GTL and create a new Profile. Exit GTL then start it again. If it's successful your cars will now be unlocked and you will have loads of cash.
Make sure your GTL is set to run in Professional mode (no driving assists except for auto-clutch which is handy if you don't have a clutch pedal).
Try to join the practise server by going on-line. Start GTL and select Multiplayer then Internet then accept the defaults (already set up previously) by clicking on the orange Log in circle on the right hand side of the screen. You should shortly see the Lobby screen. Under Game Name look for SimRacing.org.uk etc. Don't try to enter just yet.
Our races contain many third party mods e.g. tracks and cars and these are required to be downloaded and installed for every race. The Race Announcement thread (top right of Home screen under Forthcoming, first entry beginning with code letter "L", click on this) will provide download links for the track and car mods required for that race. You need to install all of the cars, even if your not planning to drive them yourself. This is so that GTL can load your opponents chosen car when you join on-line on race night. Download these and install them following all instructions provided. Once finished you can now try to enter the on-line practise server via the Lobby. Please do not wait until race night to discover you have a problem! You should test that everything is OK by trying to join the practise server before the race night thus giving us a chance to sort out your problem.
Another invaluable add-on to GTL is called XD which will provide loads of in-race information in HUD format. Things like race position, fuel used, tyre wear, fastest laptime etc are displayed for your information. Get XD here
http://www.vitumo.de/ and drop the downloaded files in to your GTL main folder.
That's just about it. Hope to see you on track real soon