You're going about this the wrong way. You wouldn't use the gform_field_value hook to populate fields on a multi-page form if the value is coming from another field on the form. You would only use traditional dynamic population methods when you are populating a field value with data that is coming from outside the form.
When populating a field value on a multi-page form with a value that is coming from another field on the same form, you have to go about things a different way. This is due to how these hooks work as well as how multi-page forms work.
You would need to use the gform_pre_render hook to do what you are trying to do. You would need to use that hook to get the value from one field and populate the value of another field using that value.
The gform_pre_render hook is documented here, there a few different examples of it's uses:
Posted 6 years ago on Tuesday November 6, 2012 | Permalink