Decrease length of underscore character in normal text (gender_gap)

I agree with ArTourter that you are probably better off simply choosing a font that has an appropriately sized underscore character. However, here's a hack using the relsize package that allows you to scale the underscore character to your liking. It would also be possible to use the underscore character only from a different font, but I'm not sure that the result is desirable, given that different fonts have different stroke weights. I've shown examples of both options in the document below:

\documentclass[oneside]{scrbook}

% (1) choose a font that is available as T1
\usepackage{lmodern}
\usepackage{relsize}
% (2) specify encoding
\usepackage[T1]{fontenc}

\renewcommand{\_}{\textscale{.7}{\textunderscore}}
\begin{document}
Forscher\_innen, Protagonist\_innen, etc.

% Original length for comparison
\renewcommand{\_}{\textscale{1}{\textunderscore}}
Forscher\_innen, Protagonist\_innen, etc.

% Underscore from Times
\renewcommand{\_}{{\fontfamily{ptm}\selectfont\textunderscore}}
Forscher\_innen, Protagonist\_innen, etc.

\end{document}

Extending Alan Munn's proposition, I finally came out with this, which does not modify \_, but still makes it right in PDFs (copying and searching work as they should)

\newcommand{\gendergap}{\kern-.5pt\adjustbox{scale={0.5}{1},raise={1.5pt}{\height}}{\textunderscore}\kern-.5pt}

Forscher\gendergap innen, Protagonist\gendergap innen

I also added these for having prettier source code:

\newcommand\g[2]{#1\gendergap #2}
\newcommand\n{"-\gendergap\relax{}in }
\newcommand\innen{"-\gendergap\relax{}in\-nen }
\WithSuffix\newcommand\n*{"-\gendergap\relax{}in} % needs \usepackage{suffix}
\WithSuffix\newcommand\innen*{"-\gendergap\relax{}in\-nen}

\g{Der}{die} Forscher\n und Forscher\innen*, Protagonist\innen und Protagonist\n*, etc

Additionally, they allow hyphenation before the _ (without it there would not be any hyphenation possibility within for example Forscher_in, which is very long). This needs \usepackage{ngerman}.

I named it \n because \in already exists (If you read the \ as an i it does even make sense :) )

Complete document:

\documentclass[oneside]{scrbook}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[ngerman]{babel}
\usepackage{ngerman} % for "- support
\usepackage{suffix} % for the star versions

\usepackage[pdftex]{adjustbox}

\newcommand{\gendergap}{\kern-.5pt\adjustbox{scale={0.5}{1},raise={1.5pt}{\height}}{\textunderscore}\kern-.5pt}

\newcommand\n{"-\gendergap\relax{}in }
\newcommand\innen{"-\gendergap\relax{}in\-nen }
\WithSuffix\newcommand\n*{"-\gendergap\relax{}in}
\WithSuffix\newcommand\innen*{"-\gendergap\relax{}in\-nen}

\begin{document}

Forscher\gendergap innen, Protagonist\gendergap innen, etc

Forscher\n und Forscher\innen*, Protagonist\innen und Protagonist\n*, etc

Line break demonstration: Forscher\innen Forscher\innen Forscher\innen Forscher\innen Forscher\innen Forscher\innen Forscher\innen Forscher\innen Forscher\innen Forscher\innen Forscher\innen Forscher\innen Forscher\innen Forscher\innen Forscher\innen Forscher\innen 

\end{document}