Proctored exams mean that there are checks included on the test taker's system which prevent the user from searching for answers on the internet or moving away from the desk during the exam. Proctored exams ensure the integrity of the certification exam as it prevent any kind of malpractices.
No, there is no negative marking for any of the GACC's exams.
There is no fixed passing percentage, and it depends on the pool of questions which you will answer. A score of 70% should be a safe score to target to ensure passing.
You can take the exam at any time when you want after the booking. However, you should access your password only 30 mins prior to starting your exam. If you begin your exam once, you attempt is counted at the start.