Because gain of function is engineered by humans while dangerous mutation is a natural occurrence.
How about man made mutation, or experimental mutation?
Either would work, and I’m sure you can come up with other terms that anyone can figure out what you mean without having to run to wikipedia.
GoF.... a despicable description for what I believe is extremely virulent and dangerous ‘research’ conducted by these people.
The reasons given for the activities are specious (e.g., to prepare for fighting possible variations of a disease) and the controls placed on these researching entities are wholly inadequate. It amounts to ‘playing God’.