drush si fails

[nateswart]

I attempted to run drush si on a previously installed site (to accept the db parameters already entered in settings.php) and received the following failure:

# drush si
You are about to DROP all tables in your 'drupal' database. Do you want to continue? (y/n): y
Starting Drupal installation. This takes a while. Consider using the [ok]
--notify global option.
exception 'Exception' with message 'PHP extensions: Disabled         [error]

Drupal requires you to enable the PHP extensions in the following
list (see the <a href="http://drupal.org/requirements">system
requirements page</a> for more information):<div
class="item-list"><ul><li class="first last">gd</li>
</ul></div>' in /var/www/html/includes/install.core.inc:768
Stack trace:
#0 /var/www/html/includes/install.core.inc(470):
install_verify_requirements(Array)
#1 /var/www/html/includes/install.core.inc(339):
install_run_task(Array, Array)
#2 /var/www/html/includes/install.core.inc(77):
install_run_tasks(Array)
#3 /usr/local/src/drush7/includes/drush.inc(708):
install_drupal(Array)
#4 /usr/local/src/drush7/includes/drush.inc(694):
drush_call_user_func_array('install_drupal', Array)
#5 /usr/local/src/drush7/commands/core/drupal/site_install_7.inc(80):
drush_op('install_drupal', Array)
#6 /usr/local/src/drush7/commands/core/site_install.drush.inc(289):
drush_core_site_install_version(NULL, Array)
#7 [internal function]: drush_core_site_install()
#8 /usr/local/src/drush7/includes/command.inc(368):
call_user_func_array('drush_core_site...', Array)
#9 /usr/local/src/drush7/includes/command.inc(219):
_drush_invoke_hooks(Array, Array)
#10 [internal function]: drush_command()
#11 /usr/local/src/drush7/includes/command.inc(187):
call_user_func_array('drush_command', Array)
#12 /usr/local/src/drush7/lib/Drush/Boot/BaseBoot.php(62):
drush_dispatch(Array)
#13 /usr/local/src/drush7/drush.php(70):
Drush\Boot\BaseBoot->bootstrap_and_dispatch()
#14 /usr/local/src/drush7/drush.php(11): drush_main()
#15 {main}