Yesterday I posted about the Gate spells in my DF game.
First thoughts on Gate spells in DF Felltower
I've made an iterative step towards how I want them to work.
Tells the caster the direction and approximate distance of one specific gate.* The caster must seek a specific gate - "the nearest gate" isn't valid, but "the nearest gate to Hell" is. A gate you haven't traveled through counts as a gate you are not personally familiar with.
Modifiers: Long-Distance Modifiers (GURPS Magic, p. 14), -2 for a closed gate, -5 to find a gate's arrival point or to find a return gate, -10 for a gate you're not personally familiar with, +5 for a physical object related to the gate (such as a key.)
Cost: 3. One try per week.
Time to Cast: 1 hour.
Prerequisites: Magery 2, Mage Sight, and one spell from each of 10 colleges including at least four Seek spells.
* I may modify this to "A success gives the caster a vision of the gate's whereabouts - or its specific direction and distance on a critical success." Not sure yet.
That should pretty much allow for seeking a specific known gate when you're close to it, but the casting time and limited tries means you're not going to throw it willy-nilly or use it to triangulate.
As written, except no smells pass through. The "open window" description means you can look at it, not lean in/through it.
Only allows you to force a closed gate open or an open gate closed. Otherwise as listed.
Some clarifications on other spells:
Mage Sight: Will detect closed gates (which otherwise aren't visible).
Analyze Magic: Will tell you the type of gate (a gate to another place in the same world, a gate to a different plane/dimension/whatever, a time gate, or a multiple), a critical success will give you a hint of the destination.
Seek Magic: This spell is expensive, largely useless, and the rare times it's useful it's annoyingly effective. So I'm eliminating it.
I think that'll work for what I want from those spells - you can find gates, but it's not easy or fast or reliable unless you know what you seek. Even then, interference is possible. You're almost better off using standard Divination magic and in-town research to back it up, which is also good. You can peer through gates but not much else, and you risk being spotted with Sense Observation. You can open and close gates. You can spot them and analyze them with magic that should do just that.
I'm still debating the points I mentioned about, but I kind of like what I sorted them down to. More gate detection and analysis once you've found one, finding one is harder, and using scry-and-decide works but without the oddness that threw us off last session.