neon tetras have a habit of disapearing. They can get sucked up the filter, or get eaten my the other fish. If the gourami is bigger then 4 inches, thenhe might be the culprit.

Also, neons have avery, very low tolerance with nitrite, a bacteria created from the breakdown of amonia. Its only reccomended to aquire neons if your tak is wel estabolished. I have customers come in all the time returning dead neons becasue my staff sells them to aqariums with massive new tank syndrome.