In reality, Google only offers 99.9% per month (99% for "reduced availability", I'm not sure what these prices are for) and the value of the guarantee is pathetic: they credit (not even refund) you a maximum of half your bill that month if availability is =95%. They could be down a full day and only knock 25% of you bill next month. That can barely be considered an SLA.
In my experience that is even better than most SLAs I've seen. Standard is to prorate the month and reduce that month's bill or add a credit to the next for the amount down so if you are paying $1000 monthly and you are down a day you get $33.40 back (assuming 30 days in the month).