Groovy/Grails Contains with Lowercase

I agree with aiolos: use constraints or try to find instance by name ignore case. But to fix this your way try *. (spread operator):

venueNameLists*.toLowerCase().contains(venueName.toLowerCase()) 

If you would like to check a duplicate entry before saving an element, use constraints on your domain class. Here you could use unique constraint or implement your own if you need it case insensitive.

If you need to check it manually, try this:

def venueWithNameFromParams = Venue.findByNameIlike(params.name) // ignore case
if(venueWithNameFromParams){
    // venueName is in venueNameList
}