This is actually quite common, I have had fish mysteriously disappear on me lots of times since I have been fishkeeping (about 40 years now). That is a small tank for that to happen, and you haven't given any information on what is in there in terms of decoration, other inhabitants, substrate, plumbing. Possible culprits include:
1. It died, as did your goby, and you your cucs cleaned them up.
2. They are super good at hiding. I have not seen fish for weeks, and then they appear. The can go under rocks, decorations, etc. Although, again, 5 gallons do not afford a lot of hiding places.
3. You have a murderer that ate them. Pistol shrimp or something. Then the cuc ate the remains. Little skull is in gravel.
4. It is in your plumbing somewhere. Tubing, reservoir, sump, etc.
5. It did jump. When fish dry out they get quite small and are hard to find sometimes.
6. It found Harry Potter's Cloak of Invisibility and is still in there.