Not knowable. The algorithm was discussed for a little while, and there was a testnet, development could have started before launch, and perhaps only a few tweaks were required at launch, if that. This is all speculation of course, but you can't say for sure how quickly a GPU miner was ready. Even to this day there are only private GPU miners. 

Even the CPU miner was poorly written and poorly scrutinized (the whole design was rushed) so the public code was very unoptimized and still is not well optimized to this day. You could say the same about CryptoNight except that within a few weeks of launch, the Monero team was releasing optimized mining code (which got copied to the other CN coins) and now even the very best coders are struggling to make big gains.


You are way off. If you understood how GPUs work you would realize this.