I can't access the guide you made, so I was wondering if I could just get some basic tips?
Sure!
Day Planning Mode is an unofficial hard mode, so it's easiest when you don't turn it on. There is some extra content on that mode, but it introduces so many other variables that it's better to save it until you've played through everyone's routes without it.
Gunnar favors strong/aggressive responses in general, but the most important thing is to meet him at whatever energy he's putting out. If he's being rude or unreasonable, call him out (though if he's arguing with another person, it's better to take his side), but if he's expressing an insecurity, reassure him.
Gunnar's the jealous type, so try to avoid any choices that lead to you spending time with the other love interests. This can be a tightrope, as it can be easy to accidentally end up on Ilma's route while doing this. The best way to avoid this is to make it very clear during the drinking game that you're not into women.
Some of this might be kind of obvious, but I tried to keep it vague so I didn't spoil anything. If you want the actual step-by-step from the guide, I can copy/paste it over here for you.
Hope that helps!