Integrate GMail login, how to deal with cross-site cookies?

Hi,

So, after seeing a lot of people here that had a huge increase in converting visitors to users by adding a Register and Login button with Gmail I've tried to do it myself.

Right now I'm facing a big problem:
Cross site cookies are blocked by default on most browsers and the only way to add Gmail login that I found is using that.

So how can I do it without in browser that have blocked cross site cookies by default?

PS: I know it's possible, as there are sites that work for me, with cookies blocked, but my site isn't working.