Why is "he will go if gareth will go" wrong? Wouldn't aiff be translated the same each time?
Not if it translates into awkward English. The section notes explain this.
In English we normally say 'I"ll go if you go' rather than 'I'll go if you'll go'.
Welsh tenses do not always translate into the same tense in English, and this is one common example.
A previous translation in this section gave "Ân nhw os aiff hi / They will go if she will go", so it does seem strange that it is given as the preferred response in one place, and marked wrong some place else.