are there gods in this games world we know to excist?

[Providence™]

I just started the DLC for heart of stone and the guy you meet at the start of the game (Mirror merchant) somehow got on the boat saying he can help me and chose the least safest way to help me and then said to meet at a cross roads. Is he supposed to be the devil?

I also hear in Skellige them talk about Odin and Norse gods do these deity's exist or is it that the people have faith that they exist?

[Reeva!]

Well his alias is Gaunter O' Dim. Clever initials. Don't know what his actual name is, but for all intents and purposes, he's pretty close.

[WrathPunch]

The Witcher world is filled with Slavic lore and mythology, hence the very runes are names of Slavic deities. Also, the various regions/kingdoms in-game resemble real places throughout Europe, so the gods in said places will reflect the beliefs of the people in those regions. Imo, the Biblical worldview makes the most sense of this: the bible speaks of "Watchers" (angelic lords) who have powers over certain regions/nations. In Dn. 10:13, there is a corrupt Watcher who prevents an angel from passing through the Persian kingdom to give a msg to Daniel. "Satans" (accusers) are another name for them, which Gaunter O' Dim reminds me of one of these figures--Lucifer being a chief Satan, hence that became his name.