Car dies when you put it into gear

Why do you need to jump-start it? Start by measuring the battery voltage with the engine off. It must be 12.6 volts. If you find closer to 12.0 volts, the battery is good but discharged. Suspect the generator which is a real common problem. If you find around 11 volts, the battery has a shorted cell and must be replaced.

Next, measure the battery voltage again with the engine running. It must be between 13.75 and 14.75 volts. If it is low, suspect the generator. It is real common on GM cars to go through four to six generators in its lifetime. To reduce the number of repeat failures, replace the perfectly good battery at the same time. As they age, they lose their ability to dampen and absorb the huge voltage spikes these generators produce due to their design.