You have to go into locations/interlagos gp open up gdb.file with note pad and find the line for tire compounds to be used and edit it.
Default tire compounds are set to (2,3) med hard you can unlock all 4 compounds by adding (0,1,2,3) and hit save before exiting
Soray it's not exactly the write wording for the tire compound line Iam doing it off the top of my head.
Edit: Here's the exact way to do it.
Tyres
Open with notepad++ (right click) the .gdb of the track your want edit. For instance here : C:\GSC2013\GameData\Locations\Putnam\putnam_champ
To allow some kinds of tyres you have to follow this syntax :
F_EXTREME_TIRES
{
CompoundsAllowed=(0,2) // A max of 4 compounds can be included in this parameter.
}
F_REIZA_TIRES
{
CompoundsAllowed=(0,2) // A max of 4 compounds can be included in this parameter.
}
The tyres code is the following :
0 : Soft
1 : Supersoft
2 : Medium
3 : Hard
In this example soft and medium are only allowed.
You can see a working example with the inerlagos track in the .gdb here : C:\GSC2013\GameData\Locations\Interlagos\InterlagosGP