Could you let me know if GeneratePress is creating more thumbnails that the one I defined on Media section? I have selected to create Thumbnails with these sizes (300px, 600px and 1200px) but in my hosting I can see more different sizes: 100px, 64px,etc.
Aah ok so the woo products. On your shop page i can see:
300w = this is the Woo Thumbnail Size
600w = this is the Woo Main Image Size
Both of these are set by Woo and can be changed if needed in Customizer > Woocommerce > Product Images. Changing these will automatically update them.
The remaining sizes: 768px, 100px, 64px are the Thumbnails that would have been set by WP – if you change them you need to use the Regnerate Thumbnails plugin to update any existing images.
Last image for example 860w would be the original image that was uploaded.