Substitution for curry powder
You can use garam masala to make curry, but it doesn't contain everything needed for a typical curry. Other ingredients are required.

Garam masala typically contains cumin, coriander, black pepper, cloves, cinnamon, bay leaf, and mace (the outer casing of nutmeg). It's a basic spice mix often used in Indian cuisine. 

To turn it into a curry, you really also need to fry some garlic, ginger, and chilli at the very least. Also, the yellow colour in western style curry powders is turmeric. Don't forget to add some salt too!