Painting dark hallway. Can I add white into gray paint to lighten it?

Yes, but I’d be tempted to take it back to where I bought it and have them shake it so it’s completely blended.

I would go the store and get the color you want - you won't be able to mix is properly.

White is not just "white" and grey is the same. You may end up with a pink hue or some other variation of something you don't like. Formulate the mixture by making up a small batch measuring well. Put it on the wall and see if that's what you want. If not, buy a color that's right for you.

Paints can be mixed if they have the same texture. You shouldn't mix a gloss, with flat. Put equal amounts of paint in a cup, and mix it, and see what color you get.

I would only do this if you have enough paint to do the whole job - otherwise you run the risk of not being able to create the exact shade if you need to buy more.

I mix a lot of my own paint to get custom colors. One thing is not to add white to dark to lighten it. You add the dark paint to the lighter paint to get the shade your happy with. In your case it would take double the amount of white paint to just start the lightening process. Make sure you mix the amount of paint you will need or more to do the job. Experiment by mixing some in small containers first.