I don't think it would be The Witcher. Nor would it be TW2 (which just had a weekend deal on Steam, ergo giving it away free would be a *major* dick move). The Witcher as a game has basically no value now, with its small price (it's about £7 now, no?) and frequent sales where it drops to under £2. So if they gave it away, it wouldn't really mean anything. Yeah, it might be a case of 20% of GOG users own it (to use the Fallout figure), but factor in the retail copies, the Steam, GamersGate, etc. copies. It's a game that has sold millions of copies.
So even though CDProjekt (RED and otherwise) are ramping up the marketing for The Witcher 3, I cannot believe they would give either Witcher game away at this point.