Which version of ENVI are you using? Are you opening the data using the top level XML of the product? For example, 'MTD_MSIL2A.xml' which is at the top level along with the manifest.safe and folders?
When I open an L1C data in ENVI 5.6, which is a TOA reflectance product, I see 1.0 set in the gains and 0.0 in the offsets. However, what is applied are the reflectance scale factors (as it is already calibrated to TOA reflectance) which are 0.0001. This allows you to convert to both radiance and TOA reflectance in floating point output.