Fault in gear box since the day i bought my new car

I bought a new mg car and there were few scratches and dents on the car on the day of delivery, but the dealer told us that it would be taken care of in a day or two. So we left from the showroom with our car and while we were on the way home we experienced that the car was having problem shifting into first gear but we thought maybe it’s a new car and we’re not used to the clutch but later that day i experienced the same problem with the gear box as it was not shifting to the first gear and complained about this to the dealer as well. So on the third day they took the car to the workshop and kept it there for 8-9 days and gave it to me. And on the day delivery they took care of all the scratches and dents but the gear box was still giving the same problem and refused to take it home with me and told them to give me replacement of a new car as this is a faulty gear box since the day i bought it. But now they’ve again taken the car to workshop and told us to wait. So what should i legally do about this?