Google forms not allowing people without gmail to respond

We use google forms for clients to fill out an intake form. Our work emails are gmail based, so the google form owner is a gmail. When we email the link to our clients, some clients with yahoo or other accounts say they are forced to sign in under a gmail account. When they try to use their yahoo or other email, it does not allow them to complete the form. When I go to settings, ALL of the options are disabled, so it should not be requiring anyone to sign into gmail. The google form is attached to form publisher, once completed by the client it sends an email to one of our work emails with a PDF version of their responses. I am unsure if that is the issue, but I cannot figure out why they are being restricted. 

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