Yes, you can start a playlist (which contains one or more sequences) via an event that is triggered via one of the GPIO pins and runs a script (as opposed to an effect). Different events can be triggered for both rising (0->1) and falling (1->0) logic levels. I assume you would want the playlist to loop, so have a look at the StartRepeatingPlaylist.sh script in the script repository for reference. You probably also want to stop playback on the previous playlist before starting the new playlist (see StopPlaylist.sh and StopPlaylistGracefully.sh for how to do this).
I am not certain the event would trigger on FPP startup as the logic level isn't changing so you probably want some other way to run a script that reads the GPIO pin state and starts the corresponding playlist independently of the rising/falling GPIO triggered events. One suggestion is to have a different playlist that is in the schedule to run continuously that contains a single script that calls the fpp binary with the '-g' flag in input mode (e.g. fpp -g <GPIO>,Input,0) which will return the current GPIO value for a given <GPIO>. The script would then use this value to terminate the current playlist and start the corresponding playlist (this could be a potential race condition that would terminate the script before it was allowed to finish). Alternatively you could have the script run from rc.local in the system init scripts.
As far as dimming the overall output based on time of day, I don't know of a way to do this with native functionality in FPP. Some controllers like the F16v2 allow you to control string and virtual string output intensity. Just brainstorming, but there is undoubtedly a programmatic way to control this externally. The only alternative I can think of is to have different sequences that are used for nighttime v.s. daytime that have the correspondingly reduced output intensities.