How do I get rid of “--” line separator when using grep with context lines?

Question

I have a text file named compare.txt where I want to extract the single line that follows every line that contains the pattern nmse_gain_constant.

Answer 1

I do this:

 grep ... | grep -v -- "^--$"

But this works too (on many, not all OS'es)!

grep --no-group-separator ...

And it doesn't spit out that "--" or even a blank line.