Filter for my 5 gallons tank?

do i need a filter for a 5 gallon tank i have a pleco or sucker fish if you dont kow what a pleco is and can i put that sucker fish in the tank and see how long i have my fish live in it if not tell me to go buy a filter or more plecos like around 4 more and some great dark spots for them and do i need gravel for the tank or is that just for show becuz someone told me that i dont need a filter for a 5 gal vtank and i wasted money on it?