Sounds like you are wanting to be able to set the G25 to closely mimic the feel of a real race car. According to what I have read in the past, there seems to be a liking of the wheel being set to around 400 degrees rotation and the in game steering lock set to around 20 or near there. You will have to play with it a little to adjust to how you want the car to feel.
As for the clutch pedal, the game doesn't require it to shift, even when you have selected to use the clutch pedal with the tick mark. The use of the clutch therefor is going to be a matter of immersion more than anything else and for that you will just have to get used to using it and ignore the fact that you can shift without it.
