Nextion gauge and DS18B20 temperature sensor with Arduino mega

[Koepel]

If you edit the title, then you can change the section of this forum. I think it should be in the "Displays" section, unless it is just a question about map().

The Nextion Arduino library has many shortcomings. That is why there are so many alternatives, which also have shortcomings. The best option is to start here:

Using Nextion displays with Arduino

Introduction Nextion displays include either a resistive touch panel (RTP) or capacitive touch panel (CTP) and provide an easy way to create a touch screen interface, or human machine interface (HMI) for your project. The displays require a spare serial port to communicate with them. To get the best from them requires an understanding of how to drive them, and what works and doesn't work. The advice here is based on my work with multiple projects using Nextion displays with both PIC and Arduino…

Then you have to find an example for a Gauge.
Perhaps it is something like this:

int t = 80;
Serial.print("z0.val=");
Serial.print(t);
Serial.print("\xFF\xFF\xFF");  

Which values needs to be mapped to which values ?
The DallasTemperature library returns a floating point number, you can cast that to a integer and then use the map().