How to replace certain blocks

I was bored on Minecraft, and so I created a Superflat world with villages. Is there any way that I can replace all of the Gravel to say... TNT? I felt like being destructive, and I was wondering if there would be a faster way than manually replacing each of the gravel with TNT by hand. I don't use WorldEdit because it sadly will not run on my computer.

This is simple to do with the /fill command. (1.8+ only)

Syntax:

/fill [x1] [y1] [z1] [x2] [y2] [z2] [BlockToFill] 0 replace [BlockToBeReplaced]

So in your case it would be:

/fill [x1] [y1] [z1] [x2] [y2] [z2] minecraft:tnt 0 replace minecraft:gravel

This will replace any gravel between the two co-ords with TNT.