Preamble for font switching

Question

I am using a specific font that does not feature italic or bold fonts (GFS Elpis). I want to keep my current font as main, and specify another font only for bold and italics. What is the preamble for this?

Here is a sample code as requested:

\documentclass[12pt, oneside, a4paper]{article} 
\usepackage{geometry}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsfonts} 
\usepackage{xltxtra} 
\usepackage{xgreek}
\usepackage{fontspec} 
\setmainfont[Mapping=tex-text]{GFS Elpis}

\begin{document}

\section{Testing}

This is a testing evironment on the GFS Elpis font (font for Greek)    and its ability to implement both \textbf{boldcase} or \textit{italics}. The font is downloaded from the Greek Font Society and it seems that it does not integrate bold or italics. Maybe the solution is to use a different font set for bold or italics? The following subsection will now test Greek as well. (See the footnotes)

\subsection{Ελληνικά}

Αυτό είναι ένα δοκιμαστικό αρχείο για την γραμματοσειρά GFS Elpis και τη δυνατότητά της να αναδείξει \textbf{μαύρα γράμματα}\footnote{this is supposed to be bold} και \textit{πλάγια}\footnote{this is supposed to be italics}.

\end{document}

Answer 1

You can use the fontspec facilities for this:

\documentclass[12pt, oneside, a4paper]{article} 
\usepackage{geometry}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsfonts} 
\usepackage{fontspec} 
\usepackage{xgreek}

\setmainfont[
  ItalicFont={GFS Didot Italic},
  BoldFont={GFS Didot Bold},
  BoldItalicFont={GFS Didot Bold Italic},
]{GFS Elpis}

\begin{document}

\section{Testing}

This is a testing evironment on the GFS Elpis font (font for Greek) and its ability to 
implement both \textbf{boldface} or \textit{italics}. The font is downloaded from the Greek 
Font Society and it seems that it does not integrate bold or italics. Maybe the solution is to 
use a different font set for bold or italics? The following subsection will now test Greek as 
well. (See the footnotes)

\subsection{Ελληνικά}

Αυτό είναι ένα δοκιμαστικό αρχείο για την γραμματοσειρά GFS Elpis και τη δυνατότητά της να 
αναδείξει \textbf{μαύρα γράμματα}\footnote{this is supposed to be bold} και 
\textit{πλάγια}\footnote{this is supposed to be italics}.

\end{document}

Note that xltxtra is not to be loaded (it used to be handy, but nowadays it just provides a couple of rarely needed features).