Gray colored smoke coming from your exhaust is usually an indicator of your engine burning oil.
Your engine could be burning oil through worn piston wrings, worn cylinder walls, bad oil valve seals etc.
If your engine has a lot of miles on it then you may need to have the engine rebuilt or if you like the truck and want to keep the truck then you can get a crate engine for around $1,000.00 or so and install it.
If you're good at rebuilding engines yourself then you could get an engine rebuild kit for around $300.00 to $400.00 for that engine.
I've rebuilt a Chevy 350 engine myself for around $600.00 in parts.
But you can buy an already rebuilt crate engine for around $1,300.00 to $1,500.00
I've driven a truck that smoked like that for around 4 years before I got tired of it and then did a rebuild of the engine.