How well does Wheelock’s Latin prepare one for reading Caesar’s Commentaries on the Gallic War? In other words, what grammatical things do I need to learn after completing Wheelock in order to read that work correctly?
Wheelock’s isn’t that great for Caesar actually. It blatantly adopts you to Ciceronian style, although for some quirky reason it teaches the Augustan forms where there is a contrast between Augustan and Ciceronian and does not go much into variations or syncopation. You will do well to take a look at alternative forms (-is for -es in third declension; -ere for -erunt in the perfect tense of verbs). Wheelock’s also omits several common forms of the subjunctive and hardly mentions syncopation. I recommend you pick up a book like Moreland & Fleischer’s and take a quick review while you brush up on a few new things. You can always just study this straight from a grammar book (like me 
), but that is quite boring and likely to make you a subject of ridicule. Footnotes or a Loeb can also compensate for Wheelock’s lackings, but some people like the independence of being able to read a passage without any help.
For those who want to try and read De Bello Gallico by Caesar after taking Wheelock’s I would actually recommend a 2nd year Latin book.
Second Year Latin by Jenney Scudder Baade Coffin ISBN: 0-205-07869-9
It actually runs you right through most of De Bello Gallico. It teaches you how to read Caesar especially. Very good book!