the ostrogoths produced a small heap of materiel to roman specifications in italy after seizing the manufactories in the province and maintaining their staff, but it was a comparatively brief period so >90% of examples are roman.

spangenhelms and ridge helmets are fundamentally the same, we're certain 'indigenous' germanic armor would generally follow a similar plan but roman techniques/quality as well as volume means the lion's share of all arefacts from late antiquity are roman. the reason is roman fabricae gilt or silvered literally every helmet which was beyond the material capacity of germanic groups. the most impressive examples recovered from germanic settings like vendel or sutton hoo (while now painstakingly recreated) were found as lumps of iron scrap in burial mounds---roman helmets are much better preserved from the protective coatings.

the association of this kind of spangenhelm (and others) with barbarians is from old archaeology and recreation art when the late roman state was poorly understood. they'd find these well-preserved pieces in distinctly germanic burials and make the reasonable assumption they had made them; but romans didn't bury their dead with arms--they were state property in fact--so it took collating the rarer armory/cache finds in roman settings with the "germanic" examples to figure out it was (almost) all actually roman.