Repetitive behaviors can include obsessive fascination with certain topics (e.g., trains, or baseball stats), rituals (having to do everything in the bedtime routine in exactly the same sequence every night, and getting disproportionately upset if anything chances to change), rigidity/resistance to change, or more classic stereotypies, like hand flapping or rocking.
Some level of rigidity and fascination with specific topics or activities is normal for toddlers and preschoolers, and plunging deeply and obsessively into certain topics is not uncommon among GT individuals. This is probably another source of confusion about ASD and giftedness.
If you are concerned about a specific child, you may consider introducing the idea of having Early Intervention take a look at him/her. There are many developmental delays that present similarly in early childhood; there is also a wide range of normal development. A team experienced with young children may be able to provide the child's guardians with some perspective on what is in the range, questionable, or concerning.
