When it comes to adventure design, how magic works really helps determine what is or is not a problem.
Spells in GURPS tend to be a lot more like reliable technology. It's not a question of vanishing resources, preparation of limited spells, or access to strange forces. Mana is usually everywhere - and in a game like DF, the areas without it or with a limited amount of it are the exceptions.
This means if you have a spell, you can cast it regularly and often. If you've got Dark Vision, then you or others can routinely see in the dark. Flight means you can fly regularly. Seek Earth means you can always check how close the nearest gold is. And so on.
At low spell levels, you'll experience some failures and costs will be an issue. But at higher levels spells get more and more trivial to cast, are often free to maintain (barring house rules), and high skill means you can keep more of them up.
So buffs, obstacle-crossing spells, detection magic (See Secrets), and protective magic (mages being routinely Invisible, using Levitate, and under a Missile Shield) are going to be on more than they're off.
This has forced me as a GM to make very different dungeons, and obstacles, and puzzles. Just some examples, familiar to folks who read our game summaries:
- Walls can be burrowed through with a steady series of Earth to Air or Shape Earth spells, so walls need to be deliberately shielded against that (and active defenses set to make the extra time needed too costly.)
- Darkness is trivially solved, so areas of darkness need to be No Mana Zones and No Sanctity as well in order to make darkness actually a thing.
- Missile traps are meaningless without surprise.
- So are most archers.
- The GM needs to know the location of all materials in all directions at all times, or at least be able to figure it out very quickly.
- Most materials can be seen through, so dungeons need to be carefully laid out if you don't want deep-scanning down.
- PCs with the right spells can give anyone DX+6, ST+6, or HT+6 to allow for good rolls - witness a knight defaulting Lockpicking to pick a difficult lock thanks to Luck and buffing magic.
That's hardly exhaustive.
But I've found it mentally useful to consider magic like technology. It's not a weird power, just a supernatural access to something routinely. Just like smartphones change what is required to strand someone in a movie or book, spells make you re-imagine what needs to be a challenge. I keep that comparison in mind. It's not unreliable strangeness as often as it's reliable results.