Guts negates the effect of burn halving your attack.
So a boost of 50% occurs to your attack, no reduction from burn at all.
It works in a similar fashion to the ability Quick Feet - that increases your speed by 1.5x when you're paralyzed, and ignores the normal speed drop which should occur.
Here's a quote from Bulbapedia in response to Luna's answer.
>The Attack drop from burn does not occur.