The consistent and clear way to solve your problem offer macro makecell from package makecell, by which you can elegantly merge first two rows in table:
\documentclass[a4paper,12pt,headsepline]{scrreprt}
\usepackage{makecell}
\begin{document}
\begin{table}
\centering
\captionabove{Caption}
\label{tab:TYPETURBSTREAM}
\begin{tabular}{|c|c|c|c|}
\hline
\makecell[b]{LONG TEXT TEXT TEXT TEXT\\
/ NEXT LINE}
& Symbol & VALUE & GRADIENT \\ \hline
A & A1 & A2 & zeroGradient \\ \hline
B & B1 & B2 & B3 \\ \hline
C & C1 & C2 & C3 \\ \hline
D & D1 & D2 & D3 \\ \hline
E & D1 & E2 & E3 \\ \hline
\end{tabular}
\end{table}
\end{document}

Addendum:
An atrctive alternative solution is use of column type which automathecaly broke cell text into more lines (as show Guilherme Zanotelli the first example in his answer). Let mi expand and simplify this example used possibilities offered by package array:
\documentclass[a4paper,12pt,headsepline]{scrreprt}
\usepackage{array}
\begin{document}
\begin{table}
\centering
\captionabove{Caption}
\label{tab:TYPETURBSTREAM}
\begin{tabular}{|>{\centering}p{6cm}|c|c|c|}% since this is not lst column, the use of \arraybackslash can be omitted ...
\hline
LONG TEXT TEXT TEXT NEXT IN MORE LINES
& Symbol & VALUE & GRADIENT \\ \hline
A & A1 & A2 & zeroGradient \\ \hline
B & B1 & B2 & B3 \\ \hline
\end{tabular}
\bigskip
\begin{tabular}{|>{\centering}m{6cm}|c|c|c|}
\hline
LONG TEXT TEXT TEXT NEXT IN MORE LINES
& Symbol & VALUE & GRADIENT \\ \hline
A & A1 & A2 & zeroGradient \\ \hline
B & B1 & B2 & B3 \\ \hline
\end{tabular}
\bigskip
\begin{tabular}{|>{\centering}b{6cm}|c|c|c|}
\hline
LONG TEXT TEXT TEXT NEXT IN MORE LINES
& Symbol & VALUE & GRADIENT \\ \hline
A & A1 & A2 & zeroGradient \\ \hline
B & B1 & B2 & B3 \\ \hline
\end{tabular}
\end{table}
\end{document}

\makecell) , etc. by the way, in the first solution (to my opinion only this has certain sense) the use of \newline for break of lines is surplus. anyway, to my opinion is better to use existed, proven and documented solution than "invent" own solution for which you need one line of (not documented) own code :) – Zarko Dec 14 '16 at 16:22\\doesn't work inside\multicolumn, it's either\newlineor a blank line. You're right, I'll add the nested tabular as another option, but my Intention was only to show that it could be done using the\multirowor\multicolumn, because it was in fact what the OP asked (If it were me, I'd use\makecell). – Guilherme Zanotelli Dec 14 '16 at 16:42