Go to Tools → options → General → Advanced

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Windows Paste the following in Text Editor command

C:\Program Files\Sublime Text 3\subl.exe %s

on a Mac type the following command

/Applications/Sublime\ Text.app/Contents/SharedSupport/bin/subl %s

On Linux type

subl %s

Goto sublime and open package manager and install gmsh-Tools

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After installation open a geo file and on the bottom right click on Plain text and go to Open all with current extension as … → gmsh.image-20210309104045408

Now you will get color coded gmsh files

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