The only way you can get to one is the guardian either allowing you to pass, or killing the guardian (which, by the way is VERY VERY HARD) Perhaps a gang up if this option seems suitable.
All guardians are neutral, but in all honesty are more 'good' than bad- this is so for example in Heno the guardian won't be so easily corrupted into letting all hell break loose (literally). Perhaps the control disease could be used on this occasion.
The gates tend to be large vortexes that look like black holes, and are linked by the guardians spirit, (so thats why only if they allow you to go only they can open it, and if they die, that part of their spirit detaches from them, allowing the new 'guardian' to use it at will.)
Also,
Just because there are 4 realms, it does not mean that there are only four divine ((or not so divine)) spirits!Ooooup, i smell a twist plot
(Darn to college also Gabag, I know what you mean. Laziness is also a justifiable problem.
)