Hello,
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.

Hello, I have ENVI 5.5 (64-bit).
When I try to open the metadata .xml or manifest.safe file I get a "file does not appear to be a valid Sentinel-2 file" error.
I was able to find the gain values in the Sentinel-2 L1C Data Quality Report for the date my images were taken and I could manually input the gain values. Does this work?
I am still having difficulties converting to radiance.
Thank you!

I believe you would need to populate the Reflectance scale factors with 0.0001 so that the conversation from TOA reflectance back to radiance can occur.