depends how thick you want it to be. 100lbs will be a thinner layer. i personally would go with 200lbs. that will give you more than enough. but if your on a tight budget (like me), 100lbs would seem ok.
I have 220lbs in my 140g. You also have to consider that different sort of gravel have different weight. So weight is not the best way to tell have much. It's better to talk about volume. In my case 15g.
It all depends on how deep you wan't your gravel bed. General rule of thumb would be 6.5 lbs per inch of gravel depth per square foot of tank bottom. So for you tank which is 10 sq ft you would need 65 lbs for a 1" gravel bed.