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For the voltmeter you could do this.

I'm trying to insert a Greek symbol/letter in my plots but can't seem to get it to work.
Actually, all you had to do was change the double-quotes to single-quotes when you set the title:
\documentclass{article}
\usepackage{gnuplottex}
\usepackage{graphicx}

\begin{document}
\begin{figure}[!ht ...

Maybe you've found a bug in the default Computer Modern math font package. Because if you switch to a different math font package, like txfonts or fourier, the infinity sign is centered along the top:

The choice of fonts can make a big difference in printed output. Generally serif fonts look better for printing, while sans serif fonts look better on a screen.

Try putting this in your preamble and see if it makes a difference (don't load the bera package if you do this):

\usepackage[T1]{fontenc ...

I followed the circuitikz example here , and it compiled fine. But upon inspecting the corners of the circuit I found that the lines were not joined properly: Here's a magnified image of what I'm talking about
circuitikz.png
That's the upper right corner, and as you can see the corner is jagged ...

Can anybody help me use one of their examples? I have installed the package.
Sure. Here's how you could make a document that includes the circuit in Figure 6 on page 17 of the MakeCirc-en.pdf documentation file.

1. Create a file called mycircuit.mp with these contents:
input makecirc;
initlatex ...

Why is better using gnuplot as two separate progrmas?
For precisely the reason you're having trouble trying to use them together. :D

It's just much easier to use gnuplot and LaTeX separately. Another advantage is that for large documents you don't have to wait a long time for the LaTeX ...

Define (a),...,(h) as coordinates , not as nodes:
\coordinate[label=left:{$A$}] (a) at (0,0,0);
\coordinate[label=right:{$B$}] (b) at (1,0,0);
\coordinate[label=right:{$C$}] (c) at (1,0,1);
\coordinate[label=left:{$D$}] (d) at (0,0,1);
\coordinate[label=left:{$E$}] (e) at (0,1,0);
\coordinate ...

If you want to use gnuplot with LaTeX, I'd recommend using them as separate programs instead of trying to call gnuplot directly from LaTeX. Gnuplot has two terminal types which produce LaTeX-friendly output: epslatex and lua. Both of those draw only the graphics parts and leave the text rendering to ...

tikZ cannot handle (in)visible lines and areas ...
It sort of can, with the transparent!0 color, though it's still slightly visible. Alternatively you could set the opacity of the line to 0.0:

that is not handled by tikZ ...
No, you're wrong. Did you even try that miniexample I gave? Here's a ...