Let
$$\begin{align}
g(\tau) &= q^{-1/60} G(q)
\\ h(\tau) &= q^{11/60} H(q)
\end{align}$$
First of all, the equalities between the hypergeometric $j$ and $k$ expressions
follow from the eulerian transformation
$${}_2F_1(a,b;c;z) = (1-z)^{-b}\,{}_2F_1\left(c-a,b;c;\frac{z}{z-1}\right)$$
Therefore it suffices to prove the identities between $g$ resp. $h$
and the corresponding hypergeometric $j$ expressions.
I will translate those to more familiar identities.
We will use the Rogers-Ramanujan continued fraction
(RRCF),
$$\rho(\tau) = \frac{h(\tau)}{g(\tau)} = q^{1/5}\frac{H(q)}{G(q)}$$
Formula $(22)$ from the above MathWorld entry on RRCF essentially states that
$$\frac{1}{\rho^{5}} - 11 - \rho^5 = \frac{1}{g^6\,h^6}$$
(Use the product representation of $g$ and $h$ to identify the right-hand side).
From this, we can easily deduce
$$\begin{align}
g &= \frac{1}
{\left(\rho - 11\,\rho^6 - \rho^{11}\right)^{1/12}}
\\ h &= \frac{\rho}
{\left(\rho - 11\,\rho^6 - \rho^{11}\right)^{1/12}}
\end{align}$$
assuming that the arguments to the radicals are small positive reals,
which should follow from the restrictions you have placed on $\tau$.
Furthermore, formula $(46)$ from the above Mathworld entry on RRCF
gives the relation of $\rho$ with Klein's $j$:
$$j = \frac
{\left(1+228\,\rho^5+494\,\rho^{10}-228\,\rho^{15}+\rho^{20}\right)^3}
{\left(\rho - 11\,\rho^6 - \rho^{11}\right)^5}$$
which allows us to write
$$\begin{align}
g\,j^{-1/60} &=
\left(1+228\,\rho^5+494\,\rho^{10}-228\,\rho^{15}+\rho^{20}\right)^{-1/20}
\\ h\,j^{11/60} &= \frac
{\left(1+228\,\rho^5+494\,\rho^{10}-228\,\rho^{15}+\rho^{20}\right)^{11/20}}
{1 - 11\,\rho^5 - \rho^{10}}
\end{align}$$
again assuming that the arguments to the radicals are small positive reals.
I need a deus ex machina now, and Raimundas Vidūnas arXiv:0807.4808v1 comes to the rescue.
His formulae $(59)$ and $(61)$
in section 6.3 ("icosahedral hypergeometric equations", p. 20)
state precisely that
$$\begin{align}
{}_2F_1\left(\tfrac{19}{60},-\tfrac{1}{60};\tfrac{4}{5};\varphi_1(x)\right)
&= \left(1-228\,x+494\,x^2+228\,x^3+x^4\right)^{-1/20}
\\{}_2F_1\left(\tfrac{31}{60},\tfrac{11}{60};\tfrac{6}{5};\varphi_1(x)\right)
&= \frac{\left(1-228\,x+494\,x^2+228\,x^3+x^4\right)^{11/20}}
{1 + 11\,x - x^2}
\\\text{where}\qquad
\varphi_1(x) &= \frac{-1728\,x\,(1+11\,x-x^2)^5}
{\left(1-228\,x+494\,x^2+228\,x^3+x^4\right)^3}
\end{align}$$
And your conjecture follows from setting $x=-\rho^5$ which implies
$\varphi_1(x) = \frac{1728}{j}$.
Summarizing, there is an algebraic relation between $j$ and $g$ resp. $h$,
and the hypergeometric ${}_2F_1$ expressions are designed to solve those
algebraic relations for $g\,j^{-1/60}$ resp. $h\,j^{11/60}$, given $j$.