1: Open up the image as a layer
This tutorial is so simple that if you just open your image, it will still be fine. Although I want to maintain a good habit and open my image as following.
File -> Open as Layers
2: Use the Fuzzy select tool
The Fuzzy Select tool (looks like a magic wand) is designed to select areas of the current layer or image based on colour similarity. This tool will help us to select the unwanted background with one click.
3: Add Transparency
An alpha channel is automatically added into the Channel Dialog as soon as you add a second layer to your image. It represents the transparency of the image.
If your image has only one layer, this background layer has no Alpha channel. In this case, to Add an Alpha channel.
Layer -> Transparency -> Add Alpha Channel
Now, set your opacity to 40% (60% transparent).