The next conversion is a little more difficult, and has more options: Magic items & (some forms of) ceremonial magic. My first thought on this is to just adapt psychotronics (Psi-Tech pp11-31) - they're TL^, so being invented long before we would expect electronics to exist is quite reasonable (and there's also the
Baghdad Battery, which may or may not have been a TL2 electrochemical cell). If the devices use something similar to electronic microcircuits, you might need psychokinesis to produce them (a Micro-PK Perk, allowing the use of psychokinesis on a very tiny scale; I gave one to
Oliver de Bruyn, in my Katalysator-2 thread, though there it was used subconsciously to produce psi boosters, edit genes, and such), as making such a thing reasonably small with medieval or earlier technology would be extremely difficult, requiring very precise jewelers at best. Even with Micro-PK and possibly some form of ESP-based Microscopic Vision, it could also take quite some time to get right, due to needing to fix mistakes that would make it useless, on top of needing to make many such items very durable. One way to do it would be to grow crystals with the circuits psychokinetically worked into them, rather than manually working with thin boards, fine gold wire, and tiny beads of doped silica and psi-reactive crystal. Either way, this does allow the GM to decide how long it takes to make & how rare the base materials are, and thus how common and expensive such items need to be.
Some of the items are fairly easy to convert: Psychotronic Batteries (Psi-Tech pp14-15) take the place of Powerstones, for example, and wouldn't need too many changes. A lot of other magic items would be specific classes of Psychotronic Generators (Psi-Tech pp13-14). The design aesthetics and user-interfaces would need to be changed to fit the setting, of course, but mechanically, they slot in pretty well. For example, a sword with Accuracy +2 and Puissance +3 could have a pair of small crystals on or in the hilt which use Probablity Alteration to make you luckier on attack rolls, and either Psychokinesis, Biokinesis, or more Probablity Alteration to increase the damage you do.
What does all this have to do with ceremonial magic? While most multi-person ceremonies should probably translate to gestalts, a few of the big ones, most especially the one that created Yrth's recurring Banestorm problem, might make more sense as single-use psionic devices with multiple operators - in the case of the Banestorm ceremony, the creators appear to have kept most or all of their notes on-site, causing those to be destroyed when the whole thing went horribly right, and thus making it even harder to fix things.
If you don't want to use psychotronics, or don't want to
only use them, there are other options. You might use Spirit Communication and social skills to convince astral entities with useful abilties to anchor themselves (or to anchor another such entity) to a physical object, forming temporary charms (GURPS Thaumatology, p139), or more permanent fetishes (Thaumatology p161 text box); you might also force them to do this, with Mental Surgery, some form of Affliction, et cetra. Another option is to Attune an object to a particular psi ability or abilites (or skill, or even a disadvantage) - the name comes from David L. Pulver's Eidetic Memory article 'Magic as Psi,' in Pyramid Vol 3 #29, pp16-18 (the issue also has a useful section on Psionic Artifacts from pp22-24, by Kenneth Peters). A selection of examples is below:
Attunement
A. Statistics: Modular Abilities (Cosmic Power; Afflictions of Permanent Psi Traits Only, -50%; Objects Only, -20%; Meta-Psi, -10%) [2/level]
or
B. Statistics: Modular Abilities (Slotted Cosmic Power; Afflictions of Psi Traits limited by number of uses instead of duration Only, -40%; Objects Only, -20%; Nuisance Effect (Traits must fit current astrological conditions), -5%; Meta-Psi, -10%) [2, +1 per level]
or
C. Statistics: Modular Abilities (Super-Memorization; Afflictions of Psi Traits with Gadget Limitations Only, -50%; Requires IQ Roll (Religious Ritual), -10%; Meta-Psi, -10%) [1, +1 per level]
('B' does not require the user to cast a horoscope, or even know how to (though the latter would be odd), as long as he can get someone else to do it. Alternatively, they might just try without it, and hope the stars are right. In case it needs to be said, as 'C' does not have the 'Physical' enhancement, it will not conjure up the gadet; just like the others, it requires the item that will become a Gadget to be present.)
Many other combinations are possible. It may seem cheap at first glance, but the +300% to make a truly permanent item is significant, as are the costs of the various abilities. Having multiple casters working together in a gestalt (Psionic Powers pp9-11) would often be necessary, as well taking significant limitations to the chosen Affliction (e. g. 'Immediate Preparation Required' and 'Trigger').
You may also want to read 'Psi-Related Gear' (chapter two of GURPS DF 14, pp23-27), although most of that is stuff that you buy or find, rather than make.
This also depends on who is filling the 'wizard' niche, which is discussed in another post.