To their credit, Gwent is doing a lot to try to mitigate the "I have to restart a card collection" problem. First off, you 'discover' the rare card whenever you open a pack (i.e. you pick from 3 choices); this allows you to avoid duplicates and focus on grabbing cards that synergize with your existing collection. And all three choices correspond to the same rarity too, so if your pack drew a legendary quality card, you get to choose from 3 legendary cards.
And while they still have individual card quality (common, rare, epic, leg), this is balanced by the fact they have card class types too (bronze, silver, gold). So while you can have as many epics or legendaries as you want in hearthstone, you can only have 6 Silver Cards and 4 Gold cards in your Gwent deck, regardless of whether they are epic or legendary.
So you can't just overwhelm your opponent with 12 gold cards; they are inherently limited and are a strategic resource in of themselves. Now yes, it's true that the legendary quality Silver/Gold cards are normally the best, but there are more than enough epic Silver/Gold cards for you to craft a viable deck because a lot of them are tech cards and those are normally epic quality. Assuming equal levels of playtime, you will have a viable collection much faster in Gwent than HS.