An indirect question is a noun clause and can indeed be used as the subject of the main clause, as required, wherever it makes sense to do so.
Here are some examples to supplement the passage from De bello gallico 3.14. (The translations aren't mine.)
Indirect question as subject of a form of esse, with a neuter adjective as complement:
primo nobis fuit dubium quid ageremus, 'At first we were in doubt what to do' (Cicero, Verr. 2.4.138)
dubium est uter nostrum sit, leviter [or leniter] ut dicam, verecundior? 'then is it doubtful which of us is – to speak frivolously – the more modest?' (Cicero, Acad. 2.126)
mirum [est] quantum illi viro nuntianti haec fidei fuerit quamque desiderium Romuli apud plebem exercitumque facta fide inmortalitatis lenitum sit. 'It is marvellous what credit was given to this man's story, and how the grief of the people and the army was soothed by the belief which had been created in the immortality of Romulus.' (Livy AUC 1.16)
As subject of a passive verb:
...non erat quaerendum cuius manu numerarentur, sed cuius iniuria cogerentur, 'the point to be inquired into was not into whose hand it was paid, but by whose oppression it was extorted' (Cicero, Verr. 2.2.26)
As subject of an impersonal verb:
quod si in philosophia tantum interest quem ad modum dicas, ubi res spectatur, non verba penduntur, quid tandem in causis existimandum est quibus totis moderatur oratio? 'But if it is a circumstance of so much moment in philosophy, in what manner we express ourselves, where the matter, and not the language, is principally regarded, what must we think of public debates, which are wholly ruled and swayed by the powers of elocution?' (Cicero, Orat. 51)
