Are there any exercises to gain height?

Nothing special: Genetics and the growth centers of your bones determine how tall you can grow.Most growth centers will close at 16-19 years of age in males. After that happens, you can not grow taller. If your growth centers are open (xray), a balanced diet/rest ; healthy physical activity would support good growth.There are no special diets, meds or exercises that make you taller.

Unfortunately not : There are no exercises to increase your height this is determined by your genes.

For posture: You can do exercises. Standing up straighter will add height. If you already have good posture, then no other exercises will add height. You might still be growing at 18 years and could still have more height ahead.