Here's my long answer: The option of retention, when used -- which is to say, infrequently -- is usually done for a couple of reasons. Repeating a grade can allow a child who is developmentally immature the chance to experience the same curriculum when his mind or body are developed enough to handle it with more success. Retention may be helpful for a child who has been absent due to illness or psychologically out of touch with learning because of a trauma of some sort, to make up lost ground. When a child who has been working at a level much lower than her potential would suggest transfers from one school system to another, some parents ask that the child repeat the same grade in order to allow him or her to build a stronger foundation before moving on to more difficult tasks. Retention is not unheard of in the upper grades, but if a student is to be held back, it's usually much easier to do it when a child is young and has not become so identified with kids of a certain age or social group.
Now, let's put this in the context of a child with learning disabilities and Attention Deficit Hyperactivity Disorder. Both of these are neurologically based conditions that require modifications in teaching style, methodology, and environment so that a child can work up to his or her fullest potential. Therefore, the only reason I can think of that a school would recommend retention would be in a situation in which a child met one of the criteria mentioned above, or if a teacher failed to make reasonable accommodations. The former situation is a tolerable justification for retention; the latter situation would be unconscionable, as it would penalize a child for the failure of a school to provide an appropriate educational environment.
And now, let's superimpose the gifted label. What purpose could be served by having any (LD or not) gifted child go through a recycled curriculum? I can't think of any. By definition, these kids can go cognitively farther and faster than other kids in their age group. If they have a learning disability, they may have a bumpier road, but they are still in the "fast lane" cognitively speaking. I can imagine that a child might have some skills that are weak, and as a result might need intensive tutoring, but this should not be accomplished through repeating a grade! A child with a learning disability may need support to enhance specific skills throughout his or her education, but should not be held back to make this happen. Unless I'm missing some important details here, I would say retention of a gifted child with LD or ADHD is simply out of the question. A great book about gifted LD kids is called Crossover Children: A Sourcebook for Helping Children Who Are Gifted and Learning Disabled, by Marlene Bireley, Ph.D.
You might be interested to know that the National Joint Committee on Learning Disabilities (NJCLD) issued a recent report which reinforces the view that neither retention nor social promotion addresses the problems faced by children who find school learning difficult. The report also suggests that providing a daily period of intensive tutoring by qualified personnel could cost half as much as retention (which has been estimated to cost about $13,000 per child), and that intensive tutoring reliably enhances achievement. (LDA Newsbriefs, Vol. 33, No. 2, March/April, 1998). You might want to get a copy of this brief but powerful article and take it with you to your next meeting at school. Nice to have the big guys on your side.