Can anyone suggest me a good Metroidvania on Gog? Or a platform with huge levels? Thanks :)

La Mulana?

This is the only thing I can think of (based on what I have heard, yet to actually play it). Might want to check out Groupees if you decide to get it.

http://groupees.com/bagb

Also, I find it weird how it is metroidvania. Metroid seems so much different than Castlevania.

The vania does come from Castlevania right?

Yup, Metroidvania or Castleroid. It comes for the similarity of the free roaming platformer between Super Metroid and all the 2D Castlevanias after Symphony of the Night.

I'm a fan of both and was about to say also, La Mulana. If you like steam, you could also get Cave Story+ (or the original Cave Story that's free!).

Duke Nukem Manhatten Project
Make no mistake it is NOT a Metroidvainia but most of the levels are huge (if you explore them rather than just go from a to b). They also have a cool 3D aspect thing where you if run in a circle or turn a corner the camera will adjust to follow. Alas, my words don't do it justice. You'll see.

And Now that my interest from the other suggestions has been peaked, how would LaMulana work on an older system (I can run Torchlight 2 pretty well) also hard doesn't bother me so long as I don't have to do ENTIRE levels over and over.

ps. My first pc game ever was Aztec. I have a soft spot for the genre.

Another one that you should already have here is Treasure Adventure Game. It gives off a simlar vibe as Cave Story, but I don't think it's quite to the same scale.

*Note: I haven't finished TAG, so I'm not sure how it stacks up to the likes of Cave Story or La Mulana.

I only played the GBA ones, but it seemed like most of the upgrades were equipment with stuff like +10 attack or whatever.
I like how Metroid gave abilities that added utility and whatnot.

Plus Fusion and Zero Mission would place markers on your map suggesting where to go, even if not saying outright where to go as opposed to having to search around finding that one place that you passed up before and can now progress in. And movement felt faster in those games.

Yes, as everyone said, La-Mulana is the closest game to a Metroidvania game you'll get now on GOG.

You can also try the FREE game Treasure Adventure Game.

May I suggest, if you have a Desura account, to try these games?
Bunny Must Die!, a japanese Metrovania game very similar to Castlevania: Symphony Of The Night, only more cute ^_^
Cave Story+, the well known indie japanese game.

If you liked the GBA Castlevanias, please do a favor to yourself and play Symphony o the Night! In my opinion, the best Metroidvania of the Castlevania saga.

The GBA Castlevanias had ability giving items, they just had RPG style equipment too.

http://castlevania.wikia.com/wiki/Harmony_of_Dissonance_Inventory#Relics
http://castlevania.wikia.com/wiki/Aria_of_Sorrow_Inventory#Ability_Souls

That was actually something that I disliked about Fusion & Zero Mission. Super Metroid had such good level design that you "knew" where to go next, without the game having to outright say where to go.
Good level design can do that, it will point you in the right direction, while making the player feel like (s)he is exploring and coming to conclusions on his/her own.

Also, the Metroidvania castlevanias had more than just "+10 attack", they gave you new abilities as you went along, thus allowing you to access new areas. This is what the Metroid games did as well.

I agree with getting these 2 games if you like Metroidvanias. Bunny Must Die is also 50% off and you can get a DRM Free download at Desura.

Telling you where to go is actually less Metroidvania-y than making you work it out. This may have been an element of Fusion and Zero Mission that you like, but it's certainly not archetypical of Metroidvanias despite those games bearing the name Metroid.

Don't get me wrong, I love those games dearly.

Thanks everyone :D I'll try Treasure Adventure and Manhattan Project (already own Bunny, CS and La Mulana). Googling a bit I also found a game called Project Black Sun on Desura. Has anyone here played it?
@tinyE: My old PC wasn't able to run TL2 but had no problem with La Mulana, you'll probably run it fine. Regarding doing levels over and over again, each area has an useful teleporter :)