Galileo's law of inertia (at least what I've learned) is
"A body moving with constant velocity will continue to move in this path in the absence of external forces".
And Newton's first law says
"A body moving with constant velocity will continue to move with the same velocity provided that the net force acting on the body is 0.
So how are these two laws different? Both laws are telling us that a body in uniform motion will continue to move in uniform motion until the net force on the body is 0.
And if they are the same thing, then why isn't Galileo given credit for the above law? We call the law "Newton's first law of motion" when in fact Galileo discovered the above law through his experiments with inclined planes.