How to spread out the letters a little bit when using text decoration?

What you are trying to achieve is called letterspacing, and one possibility would be to use the microtype package: It provides the commands \textls[<letterspacing amount>]{<text>} and \lsstyle. Since it doesn't seem to be possible to typeset text in the decoration if it appears inside an argument (or at least I couldn't find out how), only the latter can be used here:

\documentclass[border=2mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{arrows, decorations.text}
\tikzset{>=latex}
\usepackage{microtype}
\begin{document}
\begin{tikzpicture}
\begin{scope}
\draw[->, cyan!50!white, line width=1.5mm ] (6.8cm,-1.7) -- (9.5cm,-1.7) arc (-90:0:1.5) -- (11cm,1.5);
\draw[decoration={text along path, text={|\sffamily\lsstyle|gather},text align={center},raise=0.2cm},decorate] (6.8cm,-1.7) -- (9.5cm,-1.7) arc (-90:0:1.5) -- (11cm,1.5);
\end{scope}
\begin{scope}[yshift=-3cm]
\draw[->, cyan!50!white, line width=1.5mm ] (6.8cm,-1.7) -- (9.5cm,-1.7) arc (-90:0:1.5) -- (11cm,1.5);
\draw[decoration={text along path, text={|\sffamily|g{\kern1pt a}{\kern1pt t}{\kern1pt h}{\kern1pt e}{\kern1pt r}{}},text align={center},raise=0.2cm},decorate] (6.8cm,-1.7) -- (9.5cm,-1.7) arc (-90:0:1.5) -- (11cm,1.5);
\end{scope}
\end{tikzpicture}
\end{document}

gives

The letterspacing amount can be modified with the letterspace package option:

\usepackage[letterspace=150]{microtype}

where the number denotes thousandths of an em, so in this case 0.15em (the default is 0.1em). To change the letterspacing amount locally, you could define a new command:

\newcommand\ltsp[1]{\expandafter\def\csname MT@letterspace@\endcsname{#1}\lsstyle}

and use it like this: text={|\sffamily\ltsp{400}|gather}.

(Note that letterspacing with microtype works with pdflatex or lualatex, but not with xelatex.)

Here is a solution with the text effects along path option of the decorations.text library. I increased the gap, you can manage it by changing the character widths={inner xsep=1pt} option.

\documentclass[border=2mm,tikz]{standalone}

\usetikzlibrary{arrows, decorations.text}
\tikzset{>=latex}
\begin{document}
\begin{tikzpicture}[decoration={
                text effects along path,
                text={gather},
                text align=center,
                raise=0.2cm,
                text effects/.cd,
                characters={ text along path,font=\bf\sffamily},
                character widths={inner xsep=1pt}}]
\draw[->,cyan!50!white,text=black,line width=1.5mm,
preaction={decorate} ]
 (6.8cm,-1.7) -- (9.5cm,-1.7) arc (-90:0:1.5) -- (11cm,1.5);
\end{tikzpicture}
\end{document}

Here I create a tokcycle package macro \spaceouttext{gap}{text} that, when invoked, creates the desired tokens in the package's token register named \cytoks. Thus, do the invocation prior to the \draw and pass \the\cytoks as the textual argument.

\documentclass[border=2mm]{standalone}
\usepackage{tikz,tokcycle}
\newcommand\spaceouttext[2]{%
  \tokcycle{\addcytoks{##1{\kern#1}}}%
  {\processtoks{##1}}%
  {\addcytoks{##1}}%
  {\addcytoks{##1{\kern#1}}}{#2}}
\usetikzlibrary{arrows, decorations.text}
\tikzset{>=latex}
\begin{document}
\begin{tikzpicture}
\begin{scope}
\spaceouttext{4pt}{gather}
\draw[->, cyan!50!white, line width=1.5mm ] (6.8cm,-1.7) -- (9.5cm,-1.7) arc (-90:0:1.5) -- (11cm,1.5);
\draw[decoration={text along path, text={|\sffamily|\the\cytoks},text align={center},raise=0.2cm},decorate] (6.8cm,-1.7) -- (9.5cm,-1.7) arc (-90:0:1.5) -- (11cm,1.5);
\end{scope}
\end{tikzpicture}
\end{document}