I found a way to use gradient with the hue lights. Via hue integration it was not possible at all, with the zigbee2mqtt it is possible, as it comes with “gradient scenes” select entity. They are preset, but at least there are a lot of them.
This is not the implementation we hope for, but it’s better than no gradient.
It may need some workarounds and tricks. E.g. I need to turn the light on before activating a scene.