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In a music theory workbook, there was an example of a German 6th chord, which resolves to chord V. However, there is an interval of parallel 5ths in this (A♭ & E♭ going to G & D).
How are these parallel 5ths allowed or ok to do? Is it typical to do so?
You are correct that resolving the German 6th to a V results to parallel fifths (Ab + Eb -> G + D). This is one of the rare times where the parallel fifths are allowed. People refer to these specific parallel fifths as Mozart fifths.
They call them as such because Mozart did this quite often. Wikipedia provides some examples from his works and this one from his Symphony No. 39:
In the Wikipedia link on German 6ths there are some examples on how to avoid these.
Typically the German augmented-sixth chord resolves to a cadential six-four before resolving to the root-position dominant chord. Since the cadential six-four has scale-degree 3, there's no possibility of having these parallel perfect fifths between scale-degrees 3/6 and 2/5.
The Italian and French augmented-sixth chords don't have scale-degree 3 either, so they have no risk of parallels; you can resolve them freely to a cadential six-four or to a root-position dominant without issue.
Interestingly, Mozart seemed not to care about parallel fifths from a German to a V. He did this so often, in fact, that we often call such resolutions "Mozart fifths."
more about "Mozart fifths":
(translated by google)
"Mozart has more than once resolved the augmented German 6th chord with parallel fifths. He has done this so often that one may speak of "Mozart's fifths."
In recent literature is highlighted that such parallels of fifths occur in Mozart, but in his oeuvre are altogether a rarity. Examples are u. a .:
Dans un bois solitaire KV 308, T. 54-57 Symphony in D major KV 504, 2nd movement, bars 25-26, bass / viola Symphony in E Flat Major K. 543, 1st movement, bars 167-168, bass / violin II In some of the examples cited by Tappert (inter alia The Abduction from the Serail No. 16, T. 96-97), the voices are conducted in such a way that, in fact, there are no 5th parallels
However, Mozart fifths are hardly a rarity in nineteenth-century music. B .:
Frédéric Chopin: Nocturne in C sharp minor, KK IVa No. 16 (1827), m. 1. Robert Schumann: Album for the Youth op. 68 (1848), First Loss, T. 21-22. Already in 1802, Charles-Simon Catel expressly allows this kind of 5th parallels in his influential Traité d'harmonie, unless it takes place between the outside voices. [4] In German-speaking harmonies of the 20th century, it is also explicitly endorsed as the "Mozart fifths".
