What does the callout "300... MINIMUMS" mean? MINIMUMS.... 400 APPROACHING MINIMUMS...   300 MINIMUMS.   Is this a reference to gravity? Needing more speed? Or the minimums you can see to the lights on your approach to land? 

I looked this question up and someone said it had to do with being able to see the light on the runway when you are on your approach to land. But I have found that when you are in your minimums, 300 ft from ground, you need to increase your speed to avoid falling quickly or it results in a hard landing.

Do the minimums talk about gravity?
Your friend is right. The "minimums" call is done at the level over the ground at which you must have a visual reference to the runway environment in order to continue your approach. If you do not have a visual reference at minimums (due to bad weather), you have to perform a missed approach.