Hello, I'm Dr Ralston, thanks for your question.
Diarrhea or messy poo can happen in geckos for many reasons. In fact, many sick geckos will have more than one problem going on at the same time that can cause diarrhea : nutritional problems, poor husbandry, viral or bacterial infections, sand impaction, kidney failure just to name a few.
Your gecko is kind of young at 6 months old. So, I would definitely consider the mostly possibility of parasite infections. And the overwhelming most common infection in geckos is cryptosporidium.
"Crypto" is a parasite spread in the feces of geckos. It is very stable in the environment and can live a very long time. The gecko gets it from ingestion of feces from another infected gecko. For young geckos, it is likely the mother that was the culprit, or just poor husbandry conditions at the breeder.
The parasite invades the lining of the small intestine where it replicates. The small intestine is where most of the food is digested and absorbed into the body, so damage to the small intesine often leads to less absorption of nutrients and can lead to diarrhea, and weight loss.
Hopefully, you have a good environment for your gecko set up. I won't go into that here, but I want to supply you a good link. Please read that over to ensure your gecko has a nice place to live, and that the stress if not isn't contributing to this problem. (Just click here for that link)
I think this is a very good link and that it will help you.
Some geckos will stop eating entirely because they feel bad while affected with parasites. You will really need to have the feces tested. Please take a sample of the diarrhea on a wet paper towel, and place it in a plastic ziploc bag. And take this to a reptile Veterinarian to have them test it for parasites. This is the first and easiest step, and also a very cheap test as well. Your Vet will likely want to do an acid fast stain, but I would also recommend a PCR test which is more accurate for crypto. In addition the internal parasite test can also be used to find other parasites as well.
Treatment is complicated and not always successful. Hopefully, it is not crypto, but rather one of the easier treated parasites. But, it involves lifelong care of the gecko and medications that I also won't go into here, they are prescription.
But, you will really need to have that feces checked by a reptile/amphibian Veterinarian. If you do not have one in your area, you can still ask your Veterinarian to send the feces to an outside lab to have it tested for parasites - especially the PCR crypto test I have mentioned, and they would be glad to do that for you, even if they don't know a thing about geckos.
Keep in mind also, just because a Vet might not advertise they treat reptiles and amphibians, they still might have an interest in exotics in general. So, don't be afraid to call around and ask.