Am I the only one who, immediately upon reading this, pressed her legs together and went "gaps? I'm supposed to have gaps?   Where am I supposed to have gaps??"  
Wait...what?

Our kids are bombarded with images almost 24/7 of people that we are to "aspire to be." print models, commercials, Kardashians, movies, video games, and the fashion industry.  At least in today's world there are more perfect standards than there were 50 years ago so there are many more kinds of perfect to choose from!  And with the right kind of guidance, we can establish our own idea of perfect.  This is a great opportunity to explore what perfect is with your child!  Try to approach grandma's particular brand of perfect with humor if possible. It will have far less impact that way.

So many random thoughts:

When my young women friends post pretty pictures of themselves on FB, I try to respond with something like, "Looking healthy". But yeah, my first reaction is "pretty".

I wonder how you keep building confidence when they will be aware of their appearance and reactions to it. How do you say, "Yes you are pretty. Feel good about yourself, but ultimately other things are far more important?"

I suppose there's a lesson that Grandmothers aren't perfect.

Had a girlfriend a long time ago who was somewhat more into appearance/fashion than others. If she had a stray eyelash on her cheek or something, I'd brush it off and say, "Now you are perfect." A little joke about how trivial little flaws are or about how attaining perfection isn't that big a deal. And, yeah, she was.

Ultimately if you love and nurture the good things about her, I bet it will all work out. 

Here's the thing: I have so much to say on this subject that I am stunned into silence.

I was a girl. And I raised a girl. SO. Much to say.

Maybe a nice sit down with your daughter and another with your MIL telling her in no uncertain terms how that shit won't fly anymore. Good luck with both.

Not overreacting, and then there's damage control.

"Hmm, well I think perfect means it doesn't need to be different - and I don't think you need to try to be different, that's for sure.
But more than one thing can be perfect, right?
Maybe as many kinds as there are types of people? What do you think? It's like looking for Easter Eggs - how many types of perfect can we find?"

You're quite right about planting things in innocent little minds. Sounds like Grandma was delivering some old-school crazy. I'm glad she told you, so you could undo it...and may need to keep reinforcing it as time goes on (especially in the tweener and teen years).
 
I used to tell my students that people were like a flower garden - all shapes, sizes, colors, and growing at different levels. They loved that.
 
Natalie Merchant even wrote a song about it called "You Tell Yourself." Check out the lyrics sometimes.
 

Oh, my word, what an ignorant Grandma!

I have three spaces and don't by any stretch of the imagination have a "perfect body"! Flat chested, post menopause belly, thick waist ...

Seriously. Just keep countering G'ma's silliness with reality.

You and your daughter are both perfect!

No,you aren't.My last girlfriend was abused as a child.She had a nubile daughter that she protected like a lioness.Children are just that,children.They aren't toys or make up dolls,they are little tiny people trying to grow up,and depending on the big people to guide them and keep them safe.