Raw squash and other raw vegetables are very safe for dogs to eat but usually the dog can digest the squash or other vegetables much better when they're cooked.
So some dogs may get an upset stomach after eating raw squash while other dogs may be fine eating raw squash.
I would just feed the dog a little bit of raw squash at a time and see if it upsets their stomach but usually the dog will be perfectly just fine eating some raw squash.
I've fed my dog raw squash sometimes but the dog prefers the squash to be cooked and will eat it better.