Will should absolutely get a paternity test. Just because he has every faith in Gabi (and even Nick) now doesn’t mean things couldn’t change sometime down the road. And, like they said, if Nick convinces her to put his name on the birth certificate, Will would be in for some big problems and a lot of hassle trying to get it changed.
So, this is one time Will should be listening to his mother. And it would be so simple to explain it to Gabi telling her that, for genetic reasons, he wants to make sure he is named as the biological father. But, nooooo…that’s too simple.