Site will go down now and then cos of DDos attacks. During my time with GM its happened a few times.
If you notice sometimes they change the login page address to also try and get around the issue.
It will not be the first time or the last time they will get attacked.
Hashnest was being attacked earlier today (13 Dec 2015) as well, Bitcoin price goes up and people target mining sites and exchanges to see if they can break in and steal coins.
Postive thing about GM though is that they do not store your daily payouts on online wallets and get you to withdraw it like other companies do as GM pay out each day so if someone did hack in and steal coins, not really that much to try and take tbh.
Even though it would be quite a few Bitcoins, they could probably easily cover that amount and unlike others not run away and say..Ooops sorry. we lost so many Bitcoins now that we will have to close down ...Bye! etc.
It went back online a few hours afters. I agree with what you said above: their system is very secure.
What has been bothering me, though, is the incredible variance in payment times. I haven't received the payment from yesterday's mining day untill now, just for an example.
I would love some regularity with the payments...