Are you promoting the Marketo url (landing page) only or the GW url as well?
- I'm not sure Marketo collect registrations from GW url. But if it does, does your registration flow reflect that?
Another option could be:
- you're requesting more fields in GW than in Marketo and as a results leads are under registration errors. in this case you just need to manually registered them (select > change data > registered)