You may have to contact gard on here to find out for sure.  But the couple I have done nothing holds, even sanding does not work.  I put insulation panels the installed a rough cut 4x8 wood panel cut to fit and use self taping screws (for wood to metal) in the metal studs and worked well, just do not put the screws where there is no studs.

There is a lot of moisture on an uninsulated trailer wall. This will make the direct attachment of most coverings difficult to effect for a long term durability. There are construction adhesives that will work, but removing them at a later time will be very laborious. Any moisture forming between the steel and wall covering will be trapped and cause a paper surface to rapidly fail.

The glossy paint will have to be sanded to prepare the surface for an adhesive.

The best advise is what HTB described; insulating the wall first, and applying the covering to that.

Gard

Something I left out,  I did not sand the wall when I did mine.  I used liquid nails to hold the insulation panels inplace so I did not have to fight with the insulation and wood panels both at the same time.  Made much lighter work of it.  Have done 3 trailers now and all 3 still in great shape.  The first one was a 2000 bumper pull CM 3 horse slant. 

The funny thing is when I start another one, I always find something I wished I would have done the to last one.....