Using linux, plenty of choice, depends on your needs.
For a lightweight manager, as mentioned before, there is XnViewMP, does all the usual, tagging & grouping + some simple conversions if required. You can set it up to call Gimp from a selected image.

For a quick look at an image, what I can call up from my file browser is
evince, which is a general viewer. Possibly you have a similar one there already 'okular'
nview, same guy who does xnview, useful to call up several images one after the other, temporarily.
Imagemagick comes with a viewer 'display'. Useful for those more unusual file formats.

One thing to remember with linux is use of workspaces, you can have gimp open in one and quickly switch to another with an image browser open. I think ubuntu normally comes with 4 as its default setting.