The four most common gas systems for M4s in Canada are WE, KJW, KWA, and WA (G&P uses the WA system).
Read up on the pros and cons of each one. Any GBBR will likely need a velocity reducer/NPAS installed. But other than that, different systems will have different parts that you'll want to replace.
KJW is said to be "the best out of the box", and it's my personal favourite, but I haven't done much gaming with the other systems.