Shortcode Attribute not defined

If you look at the defaults for location and popTrigger they have single quotes around them…

Assuming this is correct you need to add these around the new location value, so it does not think bottom is an (undefined) javascript variable, but rather a string value, via providing 'bottom'

Or if you don’t want to modify code to add the single quotes, you could also just do: [gf_popup pause="1000" location="'bottom'"] to pass the single quotes in via the shortcode attribute.