Usually in GT games, you select the difficulty indirectly through your choice of car. If you want it more difficult, you enter with a slower car (less PP). The performance difference bonus (assuming it's actually there) discourages you from entering with cars that would make the race too easy for you. How much less PP should you enter with? Unfortunately, you'll have to experiment to find what's right for you.
The problem is that players' skill levels vary a LOT. Some may find the game challenging enough with a car at the PP limit. Others may need to drive a GT300 car against GT500 AI cars to feel challenged. In my opinion, anyone who needs to drive a car -that- underpowered to feel challenged needs faster opponent drivers. Not faster opponent cars, but the same cars, driven faster, by "better" AI drivers. I think Halo lets you select the difficulty (Easy/Normal/Heroic/Legendary ?) anytime you can get to the main menu. I think a similar system would work well in Gran Turismo.
When it comes to pit strategies... Aside from the stupidity of pitting on consecutive laps or pitting on the second to last lap, it's actually quite complicated. @.@
Ex. If you continue driving for another 3 laps, you'll be able to pit one less time, but your lap times will suffer; is it worth it?
Ex. The race is 100 laps long, and you have to pit every 11th lap to avoid running out of fuel (and your tyres are fine after 11 laps), but if you do this, you'll end up pitting on lap 99, which is kinda stupid. Do you pit every 10th lap (9 pits), or do you pit on laps 11,22,33,44,55,66,77,88 and 94 (also 9 pits), doing the last 2 stints with less fuel and softer tyres?
Maybe the developers could just hard-code an opponent's pit strategy based on experimental results. i.e. They observe that it's most efficient for a particular car in a particular race to pit twice, on laps 10 and 20 out of 30, instead of pitting once on lap 15.