The offset is to calibrate temperature between 0-100 degrees C. This is done by adjusting the temperature for each channel and bring it into line with the cold junction.
One method of achieveing this would be by using a loop and starting the offset value as 0, then compare the channels temperature with the coldjuction. If the channels temperature is below the cold junction you would increase the offset value, or the if the cold junction is above the channel temperature then you would decrease the offset value. This would keep repeating until the channel temperature get as close to the cold junction as possible. You might never get the temperature and cold junction to be exactly the same.
The gain is calculated by using a thermocouple at a known temperature, ideallly this would be a thermocouple simulator, then using the % values the process is repeated by increasing the value upwards or downwards as required. When it says opposite end of range, if you are measuring 0-100 degrees C, then the thermocouple should be at 100 degrees C.
I hope this helps.