Well as far as I can see there are are very few modifications you need to do.
1)
Magery. In Gurps magic only those with special talent can deal with complex spells. Magery comes in levels, the recommendation is to have no more than four levels in a standard fantasy setting. I would suggest a rule that states that people with less than POW 15 has no magery. At POW 15 you get Magery 1, at POW 17 you'll have Magery 2 and so on up to POW 23 and Magery 5.
2)
Learning Spells. Since the system really only works with skills per spell you'll need to have a fairly high skill base. I would suggest a base of 30 for ordinary spells and a base of 15 for very hard (difficult) spells. Allow the character to add 5 percentiles per point of Magery to the base.
3)
Spell requirements. You need to
know a required spell. I would rule that once you have skill level 40 in a spell then you understand it good enough so that you could learn spells that depend on it.
4)
Measurements. Gurps uses imperial measures. There are not that many of those in the book though. Distances are measured in hexes which is equal to one yard, so it could easily be converted to one hex = one meter. The few other instances of imperial measures are easily dealt with.
5)
Modifiers. Modifiers are given as bonuses or penalties of one or more points (Gurps uses skill levels from 1 and up). Multiply by 5 to get the equivalent BRP modifier.
6)
Skills. There's really only one skill that needs to be converted: Innate attack. It's the ability to target missile spells and other supernatural powers at targets at a distance.
7)
Range. Spells are divided into classes or broad types. Missile spells will need some kind of range rule instead of the Gurps one. I would suggest that a mage could fire a missile up to MP * 5 meters with no penalty. Above that his Innate Attack skill would be halved.
8)
Casting Time is given in seconds, use SR instead.
9)
Durations are given in minutes etc. and need not be modified.
10)
Resistance. Spells are resisted using attributes, as given in the spell descriptions. Just use CON for HT, POW for WILL and so on. You could download
GURPS Lite to get the basics of the system to help with this conversion.
11)
Damage. I would suggest ruling that spells that deal damage does 1d6 points of damage per power point used.
There's also a chapter on how to build magic items. That seems to be easy to integrate with BRP, or you could use the rules from RQ or MRQ.
That's it really.