How, under Ubuntu, can I get Wordpress past this error (GD Library not installed)?

Solution 1:

Install gd:

sudo apt-get install php5-gd
/etc/init.d/apache2 restart

And verify:

php5 -m | grep -i gd

That should do the trick!

Solution 2:

Install php GD module:

sudo apt-get install php5-gd

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Solution 3:

For PHP 7.0 use

sudo apt-get install php7.0-gd