It looks like GMail has decided to lockdown my GMail account for unusual activities, while I wasn't performing ANY operation at all: FAIL!
I've carefully reviewed what can be considered as an unusual activity and I couldn't find that having the Mailapp open and sitting in the Dock as being considered unusual.
After 30 minutes of searching around for ways to reactivate the account and following all the indications in the GMail help pages, I have figured out a way to report the problem through a web form.
As you can imagine I was getting angry by the minute and so when I've received back the automatic notification that I should check the documentation I felt like exploding. FAIL, FAIL, FAIL.
I will have to wait 24 hours for (hopefully) getting back my account. I use this account to access my job email, so this will cost me one day of work. FAIL again!
Summarizing, having your browser/Mailapp open on GMail and sitting in the Dock is an unusual activity that may lead to your account lockdown: GMail Account Lockdown: FAIL.
Update: Panic mode off! I don't know if it is the result of my cry here or on Twitter or the support email I've sent, but my I've regained the access to my account.