You're not giving us much to go on here. For specific repair information we need to know the year, model, engine and transmission of your GMC car. Please edit your question to add that information; otherwise all we can do is give you generic guesses.
Here's my nonspecific speculation: I've seen a similar problem with a VW New Beetle when it was low on transmission fluid; the problem went away when I topped it up. Have you checked the fluid level? And no, I can't tell you how to do that without the information I asked for previously.