"Sometimes when I make chicken stock it gets gelatinous. Does that mean it has gone bad?"
No, in fact the opposite is probably true! You have likely made wonderfully flavorful stock. Properly made stock becomes gelatinous from the collagen in the bones. The gelled texture has nothing to do with fat content, as many people erroneously believe. A well-skimmed stock can be just as (or even more) gelled than one which contains fat. Fat will thicken the stock, but it will not cause it to gel.
To learn more about making homemade chicken stock, watch How To Make White Chicken Stock.
Recipe: Roasted Chicken Stock