I want to get bigger pecs and arms?

Hey whats up. Well first of all, your girlfriend is right. This is because muscle is made from fat, so first you need fat. What i suggest is buying something called whey protein. Its a powder that you put in milk. The powder will help you gain muscle and the milk is what helps you with the fat intake. Whey is perfectly safe because its just powdered protein. Now about that thing you want to buy. Its a pretty cool set up and that will work great for your chest. For your arms, I would buy some dumbbells with it. Also work on pull-ups and dips. Hope that helps.