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[The awk `gsub` function](https://www.gnu.org/software/gawk/manual/html_node/String-Functions.html#index-gsub_0028_0029-function-1) takes as its first argument a regular expression indicating the substring to be replaced, and replaces a matching substring with the value of the second argument, which is the replacement string.

Since your regular expression, `/-/`, is not anchored, it will match *anywhere* in the value that is being matched against. That's why it matches the hyphens that are contained as a part of the value. Here's an example run to illustrate this:

    $ (printf '%s\n' 'blah---blah' '---blah---' 'blah------' '---------------') \
    > | awk '{gsub(/-/,"",$1)}1'
    blahblah
    blah
    blah
    
    $

(note the empty line)

To get the behavior you are after, you need to *anchor* the regular expression at both the beginning and the end of the string, and provide a regular expression that matches against that whole string. Since you want to do a replacement only of strings that consist solely of hyphens, **the regular expression should instead be `/^-+$/`** where the `^` anchors at the beginning, the `-+` specifies an unbounded but non-zero number of `-` characters (that's the `+`), and the `$` anchors at the end.

Example:

    $ (printf '%s\n' 'blah---blah' '---blah---' 'blah------' '---------------') \
    > | awk '{gsub(/^-+$/,"",$1)}1'
    blah---blah
    ---blah---
    blah------
    
    $

As you can see, the contents of a field that consists of something other than only hyphens is now preserved, but the all-hyphens field contents is replaced by the empty string.