Forgive or else!

I drive by a billboard everyday that reads:

If you do not forgive others, God will not forgive you- Matthew 6:15

Does this strike anyone else as a gun-to-the-head kind of scenario? And is it even true? Though I am hardly an expert, I have always been under the impression that if a Christian asks  God for forgiveness, it shall be granted.

No strings attached.

Apparently not, at least according to this billboard.

What I find more surprising is the thought that this billboard, with its...