Here's how to fix that issue so it's not OP in terms of healing.
~Reduce the movement speed of the healing gun carrier by 80%
~Add additional range of the gun dart (optional but still good to consider)
~Add a 1.5~2s delay after each shot taken by it.
Problem solved, that way support will be support without being OP and vulnerable at the same time, having those additional changes added will consider players when and how to use it tactically as well as for the enemy on when to push and when not to pressure considering those changes.