As per the link given here
Check the section whats new in section 1.6.1 it says
If you get undefined symbol errors, be sure to check for older versions of libpng in your library directories!
Also what did u reinstall ? a new version of GD ?

In reply to Re: '500 Internal Server Error', when using 'GIFgraph'? by mkirank
in thread '500 Internal Server Error', when using 'GIFgraph'? by Georgio

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