Let's ask Wikipedia!
Gado-gado is a traditional dish in Indonesian cuisine, and comprises a vegetable salad served with a peanut sauce dressing. It is widely served in hotels and restaurants in Indonesia, and in Indonesian restaurants in other countries. ( http://en.wikipedia.org/wiki/Gado_gado )
That's a pretty nice and concise description. What it can't show you, however, is the unfortunate side effect that peanut sauce has on everything. It's brown. It's runny. It's inevitably crap-like.
Fortunately, it's damn tasty. The sauce alone is excellent, and with a good combination of vegetables, you're set. Better yet, many international stores carry the sauce in cheap, pre-made, powdered form. Just add hot water and you've got the sauce nice and prepared. The adventurous can add spiciness using chilies or sambals.
While the usual list of ingredients is pretty fancy for those of us way out west, plenty of substitutions and/or removals can be done. Even a single-veg gado gado works great.
Traditionally, you'll want to use lots of water spinach (also "kangkung"), almost in the same way that spaghetti uses pasta. Regular old spinach works well as a substitute. Steam/blanch/etc them just to be safe.
After that, the usual additions are other green vegetables, hard-boiled eggs, cut boiled potatoes, taoge, sliced cucumbers, long beans, fried firm tofu, fried tempeh, fried shallot, some kerupuk and just about any other random stuff you want to throw in.
Serve chilled or warm, with all the ingredients freshly cooked and prepared. Refrigerates well for a few days. Freezing can be catastrophic.
And yes, in case you're wondering, this salad uses plenty of cooked/fried stuff. Remember, this dish came from a magical tropical land where food spoils quickly and illnesses concerning the digestive system are common.