If Gross Profit Margin increases over time:
An increasing Gross Profit Margin usually shows the company is been more able to control its costs while generating sales.
If Gross Profit Margin decreases over time:
A decreasing Gross Profit Margin usually shows the company is been less able to control its costs while generating sales.
If Gross Profit Margin stays the same over time:
An unchanged Gross Profit Margin usually shows the ablility of the company to control its costs while generating sales has remained the same.