urlencode error message...

No problem :)

Will the theme be updated, or will I need to add this snippet each time to use with the plugin?

If you add the PHP like this you're in the clear for theme updates: https://generatepress.com/knowledgebase/adding-php-functions/

You won't ever have to add it again :)

Yes, I understand that but I'm guessing i'll have to add this snippet for each new install I do with the GP theme, or will you be updating the core theme?

I'm not sure yet - I'm not entirely convinced that the piece of code is bad in any way. I'll look more info it :)