Can I use an Apple Watch (gps) if I’m with Koodo ?0

It will detect the fall, but if the paired iPhone is not nearby or it cannot contact it via Wi-Fi it will not be able to place the call to emergency services.

Yes. A GPS only Apple watch does not connect to cellular. So it does not matter what carrier you use with your iPhone.

Yes. If the paired iPhone is nearby or can be contacted by Wi-Fi it can use it to place the call to emergency services.