as kidonpatrol said, it's meant to be funny, poking fun at angsty songs and kind of making a sort of mockery of the record exec who told them their songs would be better that way. a "pop gem" is essentially a pop song that is considered to be, like, really great (a gem being something of high value and beauty). he's just further pushing the whole joke of this song by calling it "that one pop gem," or their one great hit as well as making it "angsty" or whatever.
as kidonpatrol said, it's meant to be funny, poking fun at angsty songs and kind of making a sort of mockery of the record exec who told them their songs would be better that way. a "pop gem" is essentially a pop song that is considered to be, like, really great (a gem being something of high value and beauty). he's just further pushing the whole joke of this song by calling it "that one pop gem," or their one great hit as well as making it "angsty" or whatever.