Sylvanus is OP not because of her statline but because of the situation she creates on the board. You opponent has to trade everyting into her or at least play inefficiently in order to remove her without risking her stealing one of his bigger minions. So it's ridiculous to compare the two, they function entirely differently.
Second, if you have to use other cards to ensure value out of Gruul, he's already become not worth it. Using stealth on him just so that he can live a turn to get in a smack at your opponents face is relying too heavily on cards that you might not have drawn at that stage in the game. Also, after he hits face so what? He'll just get hit by Execute or Shadow Word: Death or Polymorph or Hex or Sap or just a BGH the next turn. All you did was waste 2 cards to get 8 face damage which could have been achieved by taking a much simpler route with more streamlined cards.
The problem with Gruul is that he's just too slow. He hits hard, he's an amazing beatstick, no-one is denying that his statline isn't good. The problem is that he's too easily dealt with and provides no immediate value. Archmage Antonidas can generate 1-3 Fireballs on the turn you play him and more on the following which can amount to game-winning damage. Even if he gets immediately removed you still have the damage he generated in hand. Same with Alexstrasza. Even if she is removed she at least set your opponent to 15 life (dealing a hypothetical 15 damage as battlecry) or healed you to 15 life (healing for a potential of 14 life). These are all immediate effects you can benefit from. Gruul is only a beatstick, one that is outclassed by the infamous Dr. Boom who is cheaper to play and has a better body. And even Boom has more lasting of a board effect after his removal, with his Boom Bots that can swing the board state with their deathrattles.
Typically, dropping 8 mana on a big minion that gets removed the following turn by your opponent for 1-3 mana, leaving them with a free turn to develop their board, is such a huge loss in tempo that it's impossible for you to reclaim the board state and you often end up losing the game. At least in the case of Boom, he's 1 mana cheaper and leaves behind his bots that can trade into the board and deal with the enemy minions. Gruul if he's dealt with sets you a whole turn behind your opponent. If he isn't dealth with, then Dr. Boom dropped a turn earlier would have achieved the same objective and dealt an extra 10 damage (14 damage + 4 damage over 2 turns as opposed to 8 damage over one)
He's not a bad card inherently, it's just that the design space of Hearthstone is controlled by the removal spells that too easily nullify any value Gruul can produce unless you opponent just doesn't have them. In which case, you can win more easily with different big legendaries that have less risks associated with them.