The format you were using is correct. Its simply one value per element (i.e. the default .dat format).
I suspect the issue being a wrong element mapping of your data. That is, the intracellular domain (= myocard) is not equal to the extracellular domain (= myocard plus bath) and you provide the gi_scale_vec for the myocard plus bath, thus the mapping is wrong for only the myocard. With the latest commit (6db200a68), I have added a basic data dimension check to catch this kinds of errors.
For providing a parameter vector for the intracelular domain, you have to first extract the intra domain into a seperate mesh and generate a parameter vector for that mesh.
If this is not the issue, I would need you to provide me an reproducing example to investigate further.