Hi, depends what you mean by automatic - I experimented with gravel in trays this year and grew some plants in bottomless pots placed on the gravel with the green absorbent cloth under the gravel. Also put gravel on top of the soil in the pots to prevent evaporation.
The weather was pretty rubbish this year in this part of the UK but there were a few hot weekends when I went away. Using gravel like this certainly reduced how often the plants needed watering. I could go away on a Friday and not water again till Sunday or Monday. Also helped the friend who watered on my two week holiday, although she came every day to feed the cat, only needed to water stuff in the GH every other day or 4th day. Over watering isn't a problem so long as there is moisture in the tray as the compost will only take on water it can hold.
You may need to purchase trays for the grow bags to put gravel in. I used a mix of big plastic trays and wooden fruit boxes, the later lined with bin bags. The fruit boxes came free. The bottomless pots were flower buckets with their bottom cut off down to the size of the rather expensive bottomless tom pots you can purchase at GC's. The flower buckets came free too, I've at least 100 and struggle to know what to do with them
.
I'm sure there will be posters with more suggestions.
