Yeah so the question for me, what about the ranger are you seeking that you need to get those levels? Skills can be convered by a background or a feat.
Looking at the ranger's general ability list:
1) Deft Explorer: Geralt doesn't seem to be a big linguist, and while he is skilled he doesn't seem to be "Extraordinarily" skilled in a way that isn't covered by just being a higher level character.
2) Roving: Geralt doesn't seem to run particularly fast. He's very nimble (high dex) but I'm not seeing him just free climbing rocks and swimming through water like a fish (which is literally what roving is, the ability to climb and swim as easily as running).
3) Hunter's Lore: So the trick with this is, it can be replicated on the show by just having some monster knowlege lore. REally want Hunter's Lore is, is being able to identify weaknesses for a creature you have never seen and technically know nothing about it...just figureing it out out of thin air.
Geralt just seems to be very studied in a lot of monsters (aka good monster lore skills), but monsters he's never heard of he doesn't suddenly mysteriously know lot of things about them.