This paper presents comprehensive test measurements for gas journal bearings with damping structures of a bump foil layer and/or a polymer layer. A one-pad top foil forms the bearing surface, under which the bearing structure and a bearing housing are located. Test bearings include gas foil bearings (GFBs), gas polymer bearings (GPBs), and gas foil-polymer bearings (GFPBs). In addition, three metal shims were employed to create wedge effects in the GFPBs. Firstly, static load-deflection tests of test bearings estimate the radial assembly clearance. Secondly, shake dynamic loading tests identify frequency-dependent dynamic characteristics. An electromagnetic shaker provides flat bearing specimens with one-degree-of-freedom vertical dynamic loading. GFPB was measured to exhibit a higher structural damping and lower stiffness than GFB. Lastly, the electric motor driving tests examine the rotordynamic stability performance. A permanent magnet (PM) synchronous motor drives a PM rotor supported on a pair of test journal bearings. As a result, the GFPBs with mechanical preloads enhanced the rotordynamic performance with no sub-synchronous motions up to the maximum rotor speed of 88 krpm, and the bearing friction characteristics as well. Furthermore, they showed comparable rotordynamic performance to three-pad GFBs from a past literature, even with larger bearing clearances and small mechanical preloads.