There seems to be many kind. The most popular: Gel2Root. Some suggested agar, found in asian stores. Agar with nutrient would help the plant, I supposed nutrient could be found in hydroponics type accessories.
Also for Ant Farm, gel can be made by cooking Potassium Sorbate & Sodium Benzoate with some sugar.
This homemade gel for plants or Ant farm should not mold.
Unless I miscalculated the ratio of mixture, which I don't think I did, the recipe from ehow doesn't work.
However, I found that regular food gelatin (Kanten-Japanese jelly) should work with antifungal mix. Thanks to chrimn1 @ http://antfarm.yuku.com/topic/8136#.T93E05hTFRo