What is the ruling on buying a car in installments through an Islamic bank, and what should one do if it is unlawful?

It is not possible to definitively rule on dealing with Gulf Bank without knowing its nature (whether it is Islamic or usurious) and the terms of the contract. If it is usurious, it is not permissible to deal with it for purchasing a car, as it involves dealing with usury and cooperating in sin. Allah Almighty says: "O you who have believed, fear Allah and give up what remains [due to you] of usury, if you should be believers. And if you do not, then be informed of a war [against you] from Allah and His Messenger. But if you repent, you may have your principal – [thus] you do no wrong, nor are you wronged." [Quran 2:278-279]. And He says: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." [Quran 5:2]. However, if it is an Islamic bank, it is permissible to deal with it through installments if the car is in its possession and it has acquired it and sold it to you on installment without a penalty for delay; otherwise, the sale would be invalid. Giving charity is not enough to remove sin or correct an invalid contract, but one will be rewarded for it and it expiates minor sins, for Allah Almighty says: "Indeed, good deeds do away with evil deeds." [Quran 11:114]. And the Messenger of Allah (peace be upon him) said: "Fear Allah wherever you are, and follow up a bad deed with a good one that will wipe it out, and deal with people with good manners."