Millions of people suffered from Gmail's outage yesterday (April 17) globally. Reports say that the outage had lasted for 45 minutes. Myself also experienced this problem, it began when i am surfing my Gmail yesterday all of a sudden it began to show temporary error 500 then I searched for “How to access Gmail when it is down" on Google, one of these tips helped me and I was able to access Gmail. Don't know whether it works for everybody but try these tips it may be helpful.
Access Gmail in Basic HTML Version
Gmail basic Html version: Though it lacks the new Gmail feature it works when servers are down or when you've trouble logging into Gmail. Works perfectly for slow internet connections.
Use Gmail mobile version
Do you know you can access Gmail like you access Gmail in your mobile? Works in Mozilla, safari, opera browsers. But better than Gmail Html version.
Use iGoogle gadget
Lot's of users benefited from iGoogle gadget yesterday when Gmail is down. They recommended using this feature.