Here is a lively discussion of GURPS Magic 4th edition. The consensus is that GURPS Magic has some problem spells that allow players to produce materials or do things on an industrial scale. I do agree that it should be fixed but I disagree that it prevents a medieval setting from plausibly
existing using GURPS Magic by the book.
I don't ignore the issue. The fact is in rules that do "D&D" magic, where wizards have great power to create, alter, and destroy, there are always industrial applications of the spells when done in the right order. I have had several PCs who attempted to ignite a industrial revolution in the Majestic Wilderlands using magic.
The most likely outcome of any world with D&D or GURPS Magic would be some wierd Alexandrian high tech steam punk setting. When the world developed it's version of the classical Greeks it would start ramping up the technological curve.
But I don't need to worry about the probable outcome. As long as it is possible it will work for me and my players.
The main limitation is the scarcity of mages. In my Majestic Wilderlands the central regions have a population of several hundred thousand people. When I ran my all mage campaign I made a list of EVERY Mage between Viridstan and the Majestic Wilderlands. About 300+ names (yes I used a random name and gender generator for much of this).
I admit this did not include priests and clerics. There are probably 10 times the number of magic using priests and clerics than mages in my Majestic Wilderlands. However the variety of spells they can cast is very limited compared to mages. More focused on the mission of their religion.
Basically 1 in a 1000 people were full time magic users. 1 in 100 were peripherally involved in magic (potions, wise women, etc).
Mages are scarce not just because in GURPS you had to have a special trait called Magery but because somebody has to support you doing nothing but studying It takes several hundred hours to master even the most basic of spells in GURPS raw. Let alone getting up to where you can cast the Create Stone spell. Pre-Modern had limited ability to support people doing non-productive activities.
But wait, you have create food & drink and a host of other mundane spells. True, but understand that there was a point in history where magic was little more than a half-dozen spells known by a shaman focused on helping a tribe of cavemen to survive. At the centuries wore on and civilization then the full grimoire of spells began to be discovered.
Eventually there would be a point where a critical mass of knowledge and numbers is reached and a magic industrial revolution will not be just a possible result but the most likely result.
However before this points is where many campaign that do D&D or GURPS Magic can be set. That where the Majestic Wilderlands is set. The effect of magic so far has created a world I call Medieval plus 20%.
People live longer, and healthier in my Majestic Wilderlands because of magic. Isolated communities like a conclave of mages, a temple, or a king's palace use magic to greatly ease day to day living. But outside of that it still pretty much like our own medieval area. Realistically my Majestic Wilderlands is on the verge of it's Renaissance era only a century or two when progress starts accelerating.
But those two centuries give me plenty of room for regular old fantasy gaming.