Help please! is eating a lot of sugar a decent way to gain weight?

No: My first response is why do you want to gain weight? If you are very thin a metabolic issue should be ruled out. If you have decreased appetite and/or decreased food intake your anorexia should be evaluated. If you are looking to increase your caloric intake to gain weight, increase your overall calories but eat a well balanced diet. A nutritionist may help. Good luck.