The book ‘Latin prose exercises based upon Caesar’s Gallic war’ (1884) by Clement Bryans is available on the Internet Archive:
https://archive.org/details/latinproseexerc00bryagoog
I have just uploaded a scan of my copy of the KEY to this work:
https://archive.org/details/KeyToLatinProseBasedOnCaesarBryans
However note that this is the key to the second edition of the Bryans work, and not the first as available on Internet Archive. So there are 19 extra preparatory exercises in the Key that don’t appear in the Prose book, and Exercise 1 of the Prose book appears as solution number XX in the Key… and so on.
This is a meticulously prepared Prose book with a very useful section of Caesar’s vocabulary and turns of phrase at the beginning, and having the Key available should help those of us who are working through it on our own.
Salvete omnes,
doniol.