Like in any racing game, you have to be comfortable in Gran Turismo Sport to be fast out on-track. Part of being confident in what you're doing is to have the controls you want in GT Sport.
Thankfully, Polyphony have allowed you to have complete customised controls in GT Sport. Here's how you go about changing them.
When you first bott-up GT Sport, you'll be asked which default controller layout you wish to use. This is good, but if you feel like you need a change further down the line, you can manually alter these settings.
To access your controller settings, you need to select the options submenu, which is in the GT logo in the top-left of the main menu screen. Within options, press R1 to cycle to "Controllers" and then "Wireless Controller 1P", assuming that option is applicable.
From there, you'll have every button on the DualShock 4 mapped out in front of you. Each button can be assigned manually using this method. Pressing X will bring up a drop down menu of each action available. You can even set what each type of touch on the touchpad does. The level of detail here is unparalleled on PlayStation 4.
Every racer is different, but our personal favourite controller layout is the one which has accelerate and brake as R2 and L2, respectively.
If you're using a wheel, select whichever wheel you're using and assign the controls using the same method as the Dualshock 4 controller.