I created event registration web sites (you can imagine something like http://www.eventbrite.com/), which allow users to subscribe for event updates. When subscribed, we send mass emails (with the same content) to those users.
It was ok before, but recently I noticed that GMail always put the email into Spam folder. As any texts would always go to Spam folder, I suspect that my domain was blacklisted by Gmail.
1) Is there a way to request google to put my domain into the whitelist?
2) Let's say it can't and I decide to register for new domain. Is there a way to avoid the mass email to be marked as spam by Gmail? (may be something like what Facebook email notification do?)
Yes, don't send mass email :-) If you really want to avoid being considered a spammer, send out emails with less recipients, and don't swamp the mail server with them. Let's say, for example, you have thirty recipients for a given update. You can send out emails with one recipient every minute for a half hour.
Now the numbers may be different (and will of course depend on the success of your site) but the basic theory will stand up for quite a while.
As to how to get yourself whitelisted in GMail, that's really up to the recipient. They can usually do it by simply adding your email address to their contact list.
Keep in mind whitelisting there refers to individual GMail accounts, GMail itself does not whitelist IP addresses.
It does blacklist them if you misbehave but that generally means you get delivery rejects when trying to send. The fact that your messages are going in to the mail system and being delivered to spam folders indicates that this is an account-based thing, not a global GMail blacklisting of your IP/domain.
In any case, the place to report problems for GMail delivery problems is here.