gedit plugin to highlight all instances of selection

You don't need any plugin to highlight all instances of a selection in gedit. Select the text about you are interested in, then just press Ctrl+F (to search all instances of that selected text):

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Now, let me give you some explanations about those plugins. First one, smart-highlighting is working like a charm for me. How I made:

• First I checked my gedit version (from terminal with gedit --version command) - it's 3.6.2.
• Because my gedit version is 3.6.2, I downloaded smart-highlighting-3.0.5.tar.gz.
• Finally, I followed the manually installation method.

Second plugin, it's for gedit2.x and finish. It's not enough a simple hack like you tried.

[UPDATE]

Only works if installed to ~/.local/share/gedit/plugins/. More in comments.