Hi Philco,
I recently booked and purchased airfares on GOL using their call centre. I wanted to purchase one way fares and the fare selection that was coming up on their web-based fares displayed fares that demanded a return fare to be purchased. Thus the necessity to call.

The number is:
55-11-2125-3200, somewhat into the recorded message it will request 'service in english' press 3. The chap who assisted spoke excellent english (much better than my portugeuse I am ashamed to admit).

I booked my flights and paid for them with my Amex. I believe Amex is the only foreign credit card GOL will accept.

Registration:
Now, I had used their website before and had registered my e-mail (with all my information - residence, telephone number, etc. and the agent had that in front of him, which was helpful.

I don't know why the website is rejecting your e-mail address (which is your account user id with them, as you know). I note on the website there is another telephone number: 55-11-5212-0046, have never used it and do not know if they will support international calls, but it might be worth a try.

Booking Flights:

One more thing, the agent did give me my file locator at the time of booking and said the e-ticket would be e-mailed. It was approximately 24 hours before I received the e-mail so don't panic if you do not have immediate e-mail confirmation.
Finally, when I access my account it shows past, completed flights but under "future travel" it shows "no future travel bookings" and this is about a week later. HHmm, not sure about that, at any rate I have my e-ticket with confirmed flights.

Good Luck