Do I need gold medals on all missions to get 100%

In order to use any piece of Xbox Live's uploading service, you must have an Xbox Live Gold Account. It technically falls under the Multiplayer aspect, as far as I know Silver Accounts are only allowed to purchase games over Live's Marketplace, Send Messages, and at one point in time they were allowed to join Parties but I am not sure that this is the case.

Long Answer Short, Yes.

You need Gold for the leaderboards too. Anything that requires the cloud save storage needs a Gold subscription, so that includes Snapmatic pictures and the iFruit app. You can train Chop and order customisations on your iPhone/iPod but unless you have a subscription the game won't load them.

--tommysalive of GTAForums.com

Each character has their own special garage (purchasable, not the one at their houses) when you are in one there should be a prompt to press right on the d-pad to access special cars.

On my normal Xbox Gold account this works fine but on a secondary Xbox Silver account this prompt does not appear at all.

Currently the special car list only includes the Eleghy RH8 which is a Rockstar Social Club bonus car.

On the Xbox 360 version most of the Social Club content (including leaderboards, friend stats and photos) requires Xbox Live Gold so it looks like yes, you will currently require an Xbox Live Gold account to access the Special Cars list.