Color Palette for Gradient Heatmap:

1. Midnight Blue (
                            
                            #191970
                        ) - A deep shade of blue reminiscent of the night sky, perfect for representing the coolest end of a heat gradient.

2. Royal Blue (
                            
                            #4169E1
                        ) - A bright and bold blue hue, perfect for representing the mid-range of a heat gradient.

3. Deep Sky Blue (
                            
                            #00BFFF
                        ) - A soft and soothing blue color, perfect for representing the warmer end of a heat gradient.

4. Coral (
                            
                            #FF7F50
                        ) - A warm and vibrant hue of orange, perfect for representing a moderate level of heat.

5. Tomato (
                            
                            #FF6347
                        ) - A bold and bright red hue, perfect for representing the hottest end of a heat gradient. 

This color palette is ideal for creating a gradient heatmap, with each color representing a different level of heat intensity. The palette transitions smoothly from cool blues to warm oranges and finally to a hot red, making it easy to visualize the intensity of heat at different points on the heatmap.