Several reasons this could happen, and neither of them have to do with tuning. The most common one is that you didn't have as good an exit from the last turn as the others did. An example of this is the long back stretch at Monza, after turn 7.
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If you don't get a good exit from turn 7, people will pass you like you've lost a piston. Trust me, I know. Same for turns 10 and 11.
The other possible reason is that you're driving a car that should be short-shifted (shifted to the next gear well before red line), although with Gr.3 cars, I'm not sure if this applies or not - someone else can comment on this.
But the most likely reason is the first one I gave you. When I did my first race this week at Monza in the 911, several times people passed me like I had a defective car, and my first instinct was to blame the car. But I've kept using the 911, and it wasn't the car's fault - it holds its own just fine against the other cars. It was totally my fault for not getting a good exit from some of the turns.
So my mantra this week has been "It's all about the exits", which isn't 100% true, but it's helped me. I hope it helps you!