I learned it on GT4 before going there a couple of years ago. I had played it a lot on GPL but with the more accurate, present-day version on GT4, I was able to really get some of the lines down. I also spent a lot of time watching on board videos.
As for your doubt that it will help... it definitely will help in my opinion. You should have a reasonable idea of the line and a good idea of where the track goes when you get there. This means you can focus more on motorbikes and quick cars flying up in your mirrors, and on changing conditions on the track, etc.
Whatever you do though, take it easy when you get there. You should have an idea of the line, but the individual bumps aren't all accurate in GT4, and some of them will have a much bigger effect on your car in real life, the tarmac is a lot more slippery in some places than others, and in the morning, shaded areas may be frosty, etc. There is always the chance of coming round a corner to an accident as well (we saw one pretty much every lap).
If you're sensible though, you'll be fine! And knowing the track a bit makes it a lot more fun. We were driving a 1.8 litre Mercedes convertible rental (best they could give us...) and we went past 911s, Impreza STis, Escort Cosworths and Focus RSs quite often because we knew where we were going and a lot of people didn't...