What is retro gems?

I have been thinking hard about this blog. Of course I would like to make it as interesting as possible to read, but what really constitutes a "gem" of a game?
Furthermore how do you really define "retro"?

I think it would be appropriate to define retro as any game that was released to a console at least two generations past. In other words Playstation 2, Sega Dreamcast and other 6th generation console games.
Now, the last games to be released for the PS2 was PES 2014, in 2013. That's not that long ago.
Is it really fair to use the same measurement for PC games? Four years or older? I do consider GTAV to be a very modern game, although I suppose not everyone would agree. But retro? Definitely not!
Where is it reasonable to draw the line then?

And what about a "gem"?
I honestly didn't analyze the term very much when I came up with the name of the blog.
I suppose in the end it will be up to me as the author.
Most definitely really great games, and probably valuable ones as well. Interesting and unique may be covered as well, even tough they may not be the greatest of games.

For the sake of interest it would seem redundant of me to cover such well known games as Super Mario Brothers 3 for instance; everyone already know about it (don't they?).
I would appreciate your thoughts in these matters as well.

I want to apologize for being inactive for so long, and in the future I do plan to make at least one entry weekly, most likely during weekends.