The difference between 'gwnaeth' and 'gwnaiff' for the synthesised voice is so marginal it's very hard to distinguish between them.
However context is useful here, 'gwnaeth' is 'I did' and so will never be associated with the word 'yfory' so maybe it's helpful to listen out for the other words in the sentence.
'He will make Gareth tomorrow' - rather too unlikely!
The general order if words in Welsh is VSO - verb, subject, object - and this is particularly clear when using these short-form verbs. So:
- brathiff y gath y ci - the cat will bite the dog
- brathiff y ci'r gath - the dog will bite the cat