Use Conditional Logic on Dynamically Populated Field

I have a drop down that I populate dynamically. What I would like to do is set some conditional logic based on the chosen item in the drop down. The problem is I can't use the normal conditional logic since the field is populated dynamically.

So I was wondering if there was someone to manually create the conditional logic after the field has been dynamically populated? The drop down populates with event names and the value of each is a date. What I want to do is show or hide fields based on that date. This would be easy enough if the field was manually created in the backend, but the dynamic population is making this harder then it should be.

What I have done now is wrote some custom javascript to get the value of the drop down and add or remove classes on other field items, in order to show or hide them. This works but it would be nicer to keep it all in gravity forms.

I'm not sure about how/where to use gform_admin_pre_render. My form is at http://gwa.thewildlifefilmschool.com/contact-us/. I want it so that depending on which link a user clicks it populates the first question (ie http://gwa.thewildlifefilmschool.com/contact-us/?formtype=subscribe). Using conditional logic to show specific fields depending on which field is ticked. They user can then tick additional boxes if needed to show more fields. Help!