I just removed a Philips Hue Gradient Light Strip from my Hue bridge, in order to transition to zigbee2mqqt. The light strip is found, but is reporting to only support color_temp, in stead of the color_xy profile that the light strip plus has.
I found the device on the zigbee2mqtt website, and it actually also says that there’s no color on there.
Be careful that Hue created a proprietary extension to zigbee for some of the functionalities of their product (“Entertainment”).
I assume here that Gradient colors are only managed through this extension, and thus that a Hue bridge is likely mandatory.