Hi Tornado.... welcome to UKGPL.
You are more than welcome to join in the fun, although if you have not had much practice with the GPL GT cars before, you may find them a handful. Most of the GT's will be doing 1:27's or better around Mosport which gives you an idea of the pace to aim at. I presume that you have downloaded and installed all of the relevant software /updates/ mod files for GPL and have it running on your PC. I also suggest that you join one of our practice servers on IGOR to test your on-line connection before you try to use it in anger.
Samb can probably point you in the right direction for most things.
Of all the classes that we run in UKGPL the GT's ( and I include the Porsche Cup series in that) are the only ones where races are proper "Team" events. Which means that drivers have to a part of an existing team set up at the begiining of the season in order to participate. We tried using reserves last season and things got a little confusing when some reserve drove for different teams throughout the season.
I suggest that you study the rules and regs for the GT series ( here
https://www.simracing.org.uk/smf/index.php?topic=9981.0) plus the other classes, and take a look on Igor this afternoon , there will almost certainly be some of our guys using on of the servers for practice and you are more than welcome to join in and try out the GT's on-line.
I obviously have no idea of how fast you might be, nor do I know what experience you have with GPL but if you want to get some racing fun, I recommend that you start with the 65 GP cars and the very new F2's. Both of these classes generate very fast and close racing and many newcomers start with these and graduate to the 66's, 67's 69's and the GT's ( although not necessarily in that order!! or indeed to all of them).
Watch out for UKGPL practice servers on IGOR where you can join and get a feel of what level you are at. At UKGPL we always try to group drivers of the same ability into the same races so that everyone enjoys close racing with people of more or less equal ability. There is always plenty of advice available if you have any questions about car setups or anything to do with setting up GPL on your computer etc...people who are far more knowledeable than me.