My understanding is that I still own the copyright on the work but am I required to provide them the scripts that I made to generate the artwork? Can they then run the script again and claim the output as a new work?
That the software is under the GPL does not place any scripts written for it under the GPL, nor any output. The script was a tool used to create the image. he creator has copyright until/unless it is lawfully transfered to someone else. The creator is no more required to hand over the script along with the image than an oil painter is required to hand over the brush used along with a painting.