For whom?
It's important to understand a couple things.
- Players' skill varies very, very wildly. What's simple for some players will be nearly impossible for some and just right for others.
- Players on a message board like this one do not represent the average or typical player, but rather tend to lean toward the higher end of the skill spectrum.
So, when you ask if something is difficult, the answers you get depend on who is giving them. If you're asking someone that represents the top one percent, they might find an event to be fairly easy even if the bulk of players struggle to varying degrees with that same event. Also, when asking a board like this one, we're more likely to find an event to be easier than what the average player would, because on average we are typically above average.
If you're an average player, you're going to find the Formula GT event difficult, plain and simple. It's arguably the most demanding event in A-Spec (in terms of skill, disregarding the patience of sitting through an endurance race). You can take a FGT into another event against lesser cars, make some mistakes, and still win easily, but in these events you're up against roughly equal cars so you enjoy a much more limited margin for error.
Even if you go into other events and use a car that's fairly equal to the competition, you're still using cars that don't demand quite as much skill to push them to their limits, and cars that grant greater margins for error due to their more sluggish (relative to the FGT) pace. In another car you may brake for a corner for, as an example, 2.5 seconds, and if you're off by 0.1 seconds it's not a huge error, off by only 4%, but if you brake for the same corner in a FGT for just 1.0 seconds then being off by just 0.1 seconds is a much larger error, as that is now off by 10%.
Either way, it should be the most challenging event you face in A-Spec. Whether that's overly difficult, just right, or easy for you depends on your skill level. If you've found any of the other A-Spec events to be a bit of a challenge, you should find the FGT championship to be even more so.