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.