There are an infinite set of numbers are close to any number. Nothing connects these approximations other than closeness to the chosen target, and human aesthetics.
$$ \gamma \approx \frac{\pi}{\sqrt{2}} - \frac{\pi^2}{6}$$ Error: -7.08 × 10^-4 (Yours)
$$ \gamma \approx \frac{ \pi}{2 e}$$ Error: 6.48 × 10^-4 (author unknown)
$$ \gamma \approx \frac{ \ln 9 }{\ln 45 }$$ Error: -1.07 × 10^-5
$$ \gamma \approx \sqrt[4]{\frac{111}{10^3}} $$ Error: -9.79 × 10^-6 (M. Hudson)
$$ \gamma \approx .1^{-\frac{.2}{.3-4^{-\left(.5^{-\left(\left(6 \times 7 \right)^{-.8}\right)} \right)}-.9}} = 10^{- \frac{1}{3+5 \times 2^{-2^{1+42^{-\frac{4}{5}}}}}} $$ Error: -5.01 × 10^-10 (
Gerrit de Blaauw's ordered pan-digital approximation)
$$ \gamma \approx \frac{2341}{ 1492 \, e}$$ Error: -1.68 × 10^-10
$$ \gamma \approx \frac{ \ln \left( 9 + \frac{1}{4821} \right) }{ \ln \left( 45 - \frac{1}{729} \right) }$$ Error: -2.03 × 10^-11
$$ \gamma \approx \frac{ 47712}{1069 (\pi^4 - e^3) }$$ Error: -1.00 × 10^-11
$$ \gamma \approx .8^{.2674^9 - \frac{1}{3}} - .5 $$ Error: -4.29 × 10^-12 (Richard Sabey's Pandigital approximation)
$$ \gamma \approx \ln \left( \frac{3671}{17458 }+ \frac{\pi}{2} \right) $$ Error: -2.82 × 10^-12
$$ \gamma \approx \sqrt{\frac{1}{\pi} + \frac{3046}{204869} }$$ Error: 9.93 × 10^-14
$$ \gamma \approx \frac{287443}{270955 \, \ln ( 2 \pi ) }$$ Error: 6.59 × 10^-15
$$ \gamma \approx \frac{3731208726}{1587971713} - \sqrt{\pi}$$ Error: -4.65 × 10^-22