Global Scale Modeller

Due to not having a airbrush, I hand paint my models. Mainly use acrylics. Is it ok to prime the parts while on the sprue using either white or gray spray can primer?

Absolutely, though you have to deal with the missed sprue attachment parts later
Tamiya rattle can fine primer in white or grey is an awesome primer and what I used before airbrushes and Ultimate

I regulary use a car undercoat sold at my local hardware store called Canbrush it gives a light grey finish like Plasticoat paints at a third of the price and is an excelent base for whatever you throw at it next.

The above undercoat I use and any car undercoat will most probabley will be a petrolium based paint so I would also reccomend you let it cure for a couple of days so you dont get the paint bubbling reaction some people get when mixing water and oil based paints, once cured it will take any paint you want to slap on the top of it.