donglyz06

Or rather , what trim piece do I need to order for this gap?

MelWff

I see a piece of chrome trim missing on the windshield header and I see a gap looking through the windshield. I also have no idea what year the car is, your profile isn't completed and not listed in your post.
Is the interior trim screwed to the header?

SH-60B

Check the t top hardware. The bushings or pin could be worn out.

SwampeastMike

They are chrome plated castings called windshield molding corners. A bit pricey and it seems they must be ordered in pairs. That piece also holds the sun visor assembly, the front location hole for the top and the anchor point for the top latch. Frankly I don't see how the top could be installed without it. Perhaps the front of yours is broken off but the rear is still in place? The two retaining screws at the rear are easy to access; the two on the front are underneath the upper windshield trim. Be extremely careful when installing/removing the front screws as it's incredible easy to put an edge chip in the windshield. Such chips are not only impossible to repair but certain to run! The tar like substance used to seal/secure all of the metal targa top moldings is just that--tar. The actual GM product is no longer made but fibered roofing cement or blacktop path (available at any hardware store) makes a fine substitute. It comes in tubes like caulking. That sealant is a very import water proofing measure and it MUST be removed and replaced whenever a piece using it is removed. Much of it will pop off with a flat screwdriver or similar. Gasoline removes any residue.

donglyz06

It's a 1981. In new to the c3 so I'm out of my element. Here is a picture of it with the t top removed

sug

Looks like your t top seals a past there use by date .