Dropping articles in headlines is a style/guideline choice.
http://english.stackexchange.com/questions/38759/dropping-ar...You may also see question marks being dropped in translations, but that depends on the context and the language.
British and American headlines typically look a bit different as well. And there are at least 4-5 ways to capitalize a headline.
In other news (pardon the pun), "G20" typically requires a definite article. You can google your way through British and American websites to find that out (there's a general rule here too, but it's not consistently applied). IMO, either you drop both indefinite and definite articles in a headline or you don't.
The G20 is a forum of 19 countries and the EU. Its long name is The Group of Twenty.
http://www.telegraph.co.uk/business/0/what-is-the-g20-and-ho...You could also say "an assembly of governments and leaders from 20 of the world's largest economies":
http://www.sbs.com.au/news/article/2014/10/14/explainer-what...It isn't really an organization, so I wouldn't use "at" unless it's directly about the summit.
But: Lack of context
peer agreement (net): +2