\documentclass{article}

\usepackage{glossaries}

\makeglossaries

\newglossaryentry{slns} {
	name = {slns},
	type = \acronymtype,
	description = {This is the description of the glossary entry, but it acts as an acronym},
	short = {SLNS},
	shortplural = {SLNSs},
	long = {some long named system},
	longplural = {some long named systems},
	first = {\glsentrylong{slns} (\glsentryshort{slns})},
	firstplural = {\glsentrylongpl{slns} (\glsentryshortpl{slns})},
}

\begin{document}

\printglossaries

This capitalises everything of the 'long' entry: \Gls{slns}. But this one only capitalises the first letter of the 'short' entry: \Gls{slns}.

\glsresetall
The same for the plural versions: \Glspl{slns} followed by \Glspl{slns}.

\end{document}