Is there a way to know which power CTO gel you need to make the shot or is that more of an artistic choice?
Yes. There are ways to figure out what CTO you need, but what you need is really just a matter of what you want, so it's always going to be a matter of your choice.
To figure it out, you need to know what color temperature the light you have is, and what color temperature you'd like to end up with. Full CTO converts from 6500°K, like daylight or flash, to 3200°K, like incandescent light or golden hour light. So full CTO would be a good choice for making your flash look like golden hour light. If you want to simulate sunset or sunrise, the light then is even warmer at around 2000°K, so you'd want to consider a different color, probably something a little deeper orange.
You can add gels together to some degree -- if you want full CTO but only have 1/2 CTO gel handy, you can use two layers to get roughly the effect you want.
All that said, if you're using only one type of light source, like sunlight or flash or incandescent, then you don't have to worry about balancing different sources and you can go a long way just by adjusting the white balance setting on your camera (or on each image in post-processing). For example, if you set the white balance to 10000°K, then the 6000°K light that your flash emits will look very warm even without any gel at all.
The color temperature article on Wikipedia may help if you're not familiar with color temperature.