If you're refering to an acoustic guitar, yes you probably need a dehumidifier.
Once a month, dehumidify it.
Not necessary to use silica gel.
For electric guitars, due to them having a coating of finish, its highly unlikely that the wood will absorb atmospheric moisture. Except in areas where there are no finish should there be any.
Unless the finish has been completely stripped, then you'd probably have to worry a little, but I wouldn't say it'd be that bad.
Also, the humidity here is quite stable, high 80s - high 90s. So I wouldn't worry about fret sprout.
When you get a new guitar, you'd probably have to let it settle then do a setup to adjust the intonation/relief on the neck.
But after that, due to stable humidity, you shouldn't need a setup in a long time.