I read 1.5 pounds of gravel per gallon. Is it true? I need 15 pounds of gravel for my 10g?
Depends if you want it planted or not. I ignore the pounds per gallon. A 1-1.5 inch depth is less confusing.
But how much should I buy to get that depth?
Not 100% sure, but 10lbs sounds about right. Correct me someone if I'm wrong.
The gal rule is flawed because you want to cover a certain footprint, not a certain water volume. With 6 kilograms of fine gravel (grains are no bigger than 2-3 mm) I was able to get an inch depth at the front and a 2 inch depth on the back.
10lbs is what I would recommend for a standard 10 gallon. (You could obviously go heavily depending on what plants you want or the look you are going for). The pound per gallon rule doesn't always work because there are different tank dimensions for the same gallon tank i.e. a 20 gallon long vs. a 20 gallon tall. For a standard 10 gallon tank you are fine with the pound per gallon rule.