Problem in drush configuration with wamp server 64 bit
PHP for Apache and CLI have different configuration files, so check if you're using the right one.

To identify which php.ini is used by drush, run drush status:

$ drush status 
PHP executable         :  /path/to/bin/php                         
PHP configuration      :  /path/to/php.ini


then edit that file and check if you've the right extensions specified.

Alternatively check using php command, for example:

$ php -i | grep php.ini


Note: In case grep command is not found, replace with the with find.

If you still have a problem, possibly that your DLL library was compiled for the wrong version of PHP. So either use the one included with WAMP, or download the right one specifically for that version of PHP.

Also double check that your command-line php points to the right place (WAMP folder):

$ where php
$ which php


which should give you C:\WAMP\PHP\bin-like path, if it's not, update your system PATH variable in advaned system settings.

Finally to check if your GD2 library is loaded, check by:

php -r "phpinfo();" | grep gd2


Note: The grep can be replaced with find on Windows if you don't have Linux command utilities installed.