Grandparent alienation syndrome, sometimes referred to as GAS, is a term spun off from the term parental alienation syndrome, or PAS. In a 1985 journal article, psychiatrist Richard A. Gardner used the term parental alienation syndrome to describe a process in which a parent "programs" a child to reject the other parent, usually during the course of a custody battle. The child is rewarded for acting antagonistically toward the targeted parent and for devising his or her own charges against the targeted parent.
Some grandparents and grandparenting groups have adapted Gardner's ideas and created the term grandparent alienation syndrome to describe a scenario in which a child is programmed to reject a grandparent. This programming can be carried out by either or both parents. The most prominent group using the term GAS is Alienated Grandparents Anonymous, or AGA, which is based in Florida but has outreach operations in many states.
Neither PAS nor GAS has been widely recognized in the psychiatric community. The fifth edition of the Diagnostic and Statistical Manual (DSM5) does not mention either syndrome. PAS has been cited in some court cases in the United States, but it has been challenged on both legal and scientific grounds.
If you are interested in reading more about PAS, the Open Directory Project lists a number of scholarly articles about PAS. It appears that GAS has not been the subject of scholarly research.
Those who are having difficulty with access to grandchildren may be interested in these articles: