This is an example of the gform_field_value function

gform_field_value_{your_field}

/**
 * Pre-populates a company ID into a Gravity Forms field from a GET variable
 *
 * @since   0.1.0
 *
 */
add_filter('gform_field_value_company_id', 'company_id_population');
function company_id_population(){

  // if we have the proper company GET variable - set the return variable
	if ( $_GET['company_id'] ){
		return $_GET['company_id'];
	}

}

This doesn't seem to work for me.

be sure to check the 'allow dynamically populate' field for this field in the form itself
and give it the param-name (in this code-case) : company_id

(gform_field_value_company_id) in where company_id is the parameter you fill in

Still doesn't seem to work.

You have to call a predefined "populate_XXX" function and not use another naming.