Are the computer software for biofeedback for add children just a gimmick?

Some are. In the 70's the gizmo was a $20, 000 piece of equipment with rows of " pac-man." if the child gazed steadily at the screen, the "pac-men" gobbled each other up. Children with adhd can " hyperfocus" on tv or video games the same way, especially if they're fast-paced & provide immediate reinforcement. This does not generalize to real-life situations nor improve ability to inhibit one's own behavior.
COMPLEX ANSWER. Some fuse biofeedback with neurofeedback-they are different. Simpler and well researcher bio forms can add to other treatments. In very mild add, esp. Add with anxiety it can be of use. But it takes time. Neurofeedback is direct chemical alterations of pathways through special types of exercises. In some it appears to help with a serious expert. Like religion, your philosophy of healing matters.