After Level 20...

...can you gain Attribute points?

Well, it depends on your progress in the game. You'll stop earning attribute points from levelling up, but you can gain them through quests (Tyrian/Prophecies and Canthan/Factions) or gaining Sunspear Ranks (Elonian/NightFall). You might want to check Attribute Points on GuildWiki for more info.

Once you hit level 20, you will have gained a total of 170 out of a possible 200 attribute points strictly from leveling. The other 30 points are earned by completing 2 quests that occur in different locations depending upon the game you are playing. If you've earned the full 200, you will not earn any more.

I suggest you do those quests, because pretty every build becomes less useful without those last 30 Attribute Points.

Can I decrease levels?

no you can't decrease levels

Edit: Whoa I'm an Ascalonian Squire now.

No, you can't. I'm guessing you ask because you want to decrease your attribute point distribution? Well lucky for you, you can do that whenever you're in a Town or Outpost.

I'm a Forge Runner. The "titles" don't really mean much.

Thank you so much!

It seems your total post count is at 25. Did you used to have more?

If your from Tyria, you have to do the quests that unlock more second proffessions
Cantha...not sure
And for Elona get to a certain Sunspear rank and talk to that Dude

Factions is also done through quests, but you can get them fairly early on Istan, the beginner island instead of somewhere around/past half of the game like in Prophecies.