Imperial Gunners are rumoured to have used 3 types of front armour.
1, Black Sandtrooper Pauldron worn down the chest
2, Leather jacket
3, Armour similar to TIE Pilot.
Most 501st guys opt for the cheaper Pauldron look, as to me it seems the most accurate.
As a matter of fact, myself & the missus put in our Gunner applications to the UK Garrison & 501st last week, and were cleared with no probs at all.
Here's a pic of me, to give you an idea.
HTH.