I got Fallout 4 from G2a, but now i fear that i can't Pre-load it??

It would depend on when you get your key, which in turn depends on the seller. If you get it early, then yes.

Also, it is generally a bad idea to buy from grey market resellers like G2A for several reasons, the main being that you have no guarantee that the key you get will be legit or obtained through legitimate means. I would suggest reading http://www.polygon.com/2015/2/9/8006693/the-truth-behind-those-mysteriously-cheap-gray-market-game-codes article as it contains quite a bit of information on the subject. If you still want to use grey market sellers afterwards, go for it, but at least you'll be well informed. (All this is based on the assumption that you did not already know this. If you did, disregard most of what I said).

G2A is not a grey seller or even resellers (this depends if you are buying direct from them or from other plp inside their site, you can choose)...and they give full guarantee. but Im not sure about the preload.

Those key sellers send the keys after release, they cannot do that before so no pre-load.

Thanks guys/girls hehe, well then i will just have to wait i guess

I did a quick bit of reading and this seems to be correct. I also have to add that I have bought several keys from G2A and have never had any issues, but others have had different experiences.