Newby Question - Select by Color Tool

[Ofnuts]

Yes, what you want is more or less a "luminosity mask":

* Make a B&W copy of the image
* Use the Threshold tool to set to white the values that are within a given range and to black everything else (the Threshold tool has two handles, so you can define a range with it).
* Use the result as a selection

In practice,

• The script linked above will create the B&W copy of the image as a "Channel" ("Raw luminosity")
  
• You apply the threshold tool to said channel
  
• Right click the channel and "Channel to selection".
  

A slower way is to:

• Duplicate the layer
  
• Color>Desaturate
  
• Apply the threshold tool
  
• Open the Channels list
  
• Right click any of the R/GB channels and Channel to selection".
  
• In the Layers list, hide/delete the B&W layer.