All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
"Rayan" is an adjectival participle of the Arabic verb "Rawaya" which, basically, indicates drinking water to one`s fullness. It is used to refer to being full in terms of intangible matters. For example, one who has reached their full from knowledge is called a knowledgeable person and the like.
For more details, kindly refer to {Mo`jam Maqayees Al-Lughah, vol.2/pp.375} by Ibn Fares (May Allah have mercy on him).
Accordingly, there is no sin in naming the newborn "Rayan." Rather, if it is understood that this name indicates that the named is filled with knowledge, religion, morality or being optimistic that this newborn will be a gate to goodness. In addition, one of the gates of paradise is called "Rayan". Insh`Allah, it is one of the good names and history indicates that many scholars of Hadith and Islam had the name "Rayan." And Allah the Almighty knows Best.