If you missed GT Sport and this is your first foray into Sport Mode, I would definitely jump in (and keep in mind that the terrible multiplayer stability is hopefully fixed soon). You will get a Gr 4 and Gr 3 loaner car from the Manufacturer that you can using for the duration of the season.
If you haven't played Sport Mode before, you are likely at DR/SR level D/E which means you will also be with the other "noobs", but those "noobs" will be a mix of people brand new to sim racing, long-time GT players who skipped GTS, and some other experienced sim racers who skipped Sport. So you will likely see a lot of erratic lap times in your split.
It's a good moment to wrap your head around Sport Mode and get used to how things work. I would use this test season to get some experience and begin putting your DR closer to your natural level in preparation for the real season. Get your loaner cars and do an hour practicing Deep Forest from the event menu. Then get into a slot and enjoy the race. Although if you are from Czech Republic you might have already missed today's races. Also Daily Race A is good for beginners to get up to speed. Between these four test events and a few Daily Race A's you should be in pretty good shape for when the real season starts.
It's a lot of fun!
But the game needs major bugfixes.