Issue: GARS conflict when moving wiki article

I'm guessing GARS is not ready for includes/functions_newpost.php function build_new_post to be called except on newthread.php or newreply.php. This is an issue with GARS. It should be checking that the function is defined or it should be calling its definition files regardless. As a plugin on a function it should be reusable just like the function it's inside.

How do I get rid of this error?

Find the GARS plugin related to hook location newpost_complete and find out why it runs only for wiki pages. Since the class GARS_base seems to exist, but the method persist_values doesn't, then there is something fundamentally wrong with the plugin, and maybe it should just be disabled. If GARS_base doesn't exist, and your error message just doesn't bother mentioning that, then you should probably wrap the plugin with a class_exists() condition.