Save the shell script from the link
https://mail.gnome.org/archives/meld-list/2008-April/msg00010.html
into a file called ~/bin/svn-diff-meld.sh:

#!/bin/sh

# SVN Diff Wrapper for Meld

# KOG 2008-02



left="$6"

right="$7"



meld "$left" "$right"

Then make the file executable via chmod +x ~/bin/svn-diff-meld.sh.

Create ~/.bash_aliases and paste in

alias svndm='svn diff --diff-cmd=~/bin/svn-diff-meld.sh'

but remember to replace tilde ~ with your absolute path because it needs to find the file e.g.

alias svndm='svn diff --diff-cmd=/home/arbartar/bin/svn-diff-meld.sh'

if your username is arbartar.

Run ~/.bashrc to update aliases.

Now you can run svndm some_directory/some_file within your local SVN and the GUI would pop up.