Enter your current GPA, the credits you have already earned, the credits still to come and the cumulative GPA you are targeting. The required average appears immediately, with a note on whether it is realistic.
Your current grade points equal your current GPA times the credits you have completed. The tool works out the total grade points your target needs across all credits, subtracts what you already have, and divides the shortfall by the credits you have left.
Then the target is not reachable on a 4.0 scale within the credits remaining, and the tool says so and shows the highest cumulative GPA you could still finish with. Taking more credits is usually the only way to change that.
Yes. Set the maximum to 5.0, 10.0 or whatever your institution uses and the arithmetic and the reachability check both follow that scale.