The first issue here is that the files you provided are not valid gzip format files, which explains why Surfice, FSLeyes, MRIcroGL, etc can not read them. Specifically, they have the gzip header and lz77 data stream, but omit the gzip footer. So the first task is to uncompress these to simple .nii files (and optionally recompress them to proper .nii.gz files). 

You can then use Surfice's Advanced/ConvertVolxelwiseVolumeToMesh to turn your hippocampus mask into a triangulated mesh. You need to choose a reasonable intensity for your isosurface boundary. Finally, you can load you TFCE statistical map as an overlay.