"Unentgeglich" is a word I can't find in dictionaries but have heard used. I think I have spelled it right. Can it be used to mean something is cost free?
Unentgeltlich (your spelling is a bit off) is the most high-level expression to say something is "for free". It is often used by public administration, you nearly never hear it in everyday speech. Usability is also a bit limited because its only use is like in
Sie erhalten eine unentgeltliche Probepackung
Die Probepackung ist unentgeltlich
is gramatically right, but sounds very odd.
Kostenlos is more on the everyday side, means the same.
Both are used exclusively used saying that something does not cost money
The third one in the range, is umsonst, which can mean that something is for free, but has a second meaning of "in vain" or "useless" - The danger with this word is illustrated by
Meine Schule war kostenlos - Deine anscheinend umsonst
(Could mean both my and your education was for free, but rather targets at the second meaning, saying my education was for free, yours useless)
Despite of this, the last one is the most often used to say something is for free.
"Unentgeltlich" is related to German "gratis", which matches the "old school" meaning of "without compensation expected", but which has little to do with how "gratuitous" is commonly used in the english language today (eg in "gratuitous sex and violence", implying a negative judgement) - which would, depending on context, be best translated by "sinnlos", "überflüssig", "verschwenderisch", "willkürlich".