GIMP (GNU Image Manipulation Program) is a free and open-source raster graphics editor used for image retouching and editing, free-form drawing, converting between different image formats, and for more specialized tasks. GIMP is released under GPLv3+ licenses and is available for Linux, macOS, and Microsoft Windows. Here we shall talk how to apply transparent background in Gimp, the most popular open source alternative for Photoshop as found in Linux and FOSS world !
Select Layer → Transparency → Add Alpha Channel
Select the background using the Fuzzy Selection Tool (Magic Wand)and press the Del key.
GIF will allow you single-color transparency, while PNG has full alpha channel support.
OR
Select: Layer → Transparency → Add Alpha Channel
Select: Background with Fuzzy Selection Tool (Magic Wand)
Select: Layer → Transparency → Threshold Alpha
Set Threshold to 255 for complete transparency.