This game focused primarily on online racing and did not include an extensive single-player campaign like previous games in the series.
But this is not true at all. It’s true that GT Sport didn’t have a lot of single player content initially, but that was resolved within a year of release with a massive load of free patches. The game is now full of single player content. So much so that it is not inferior to the previous titles.
In addition to the arcade mode, there is a very extensive driving school (48 missions), mission challenges (64 missions), track experience (every track, including DLC) and a GT League with a large number of races (301!), from short beginner races to approx. 24 hour endurance races. And it’s all free of charge. Then there is Lewis Hamilton’s (paid) challenge. There is also a very comprehensive photo mode, in both in-game and private locations.
Even in sports mode, you will find many players who never race online, but focus only on the weekly qualifying leaderboards.
In total, there are approximately 340 cars in the game, of which about 50% have been added as free DLC and you can all unlock them via single player.
What’s missing is extensive tuning, at least performance tuning in the game, just buying this is greatly simplified. There is also no external tuning (stickers of course, etc., but no spoilers, dividers, etc.). There is also no second-hand market.
If you look purely at the number of hours of racing and challenges in the game, it is very close to other Gran Turismo games. For comparison, the GT6 had about 230 league races, the GT Sport up to 301, and in general the races in GT6 are shorter than that.
I can highly recommend every player to get a GT Sport for almost nothing. The game has been regularly reduced to $10, and it still looks great without dropping a frame below 60, so it has enough SP content to keep you occupied for weeks.