John Pare' said:
Also, in Glorantha, to be a sorcerer, do you have to be a God Learner?
Short answer: No.
Followers of Malkion (the monotheistic religeon of the Middle Sea Empire a.k.a. "God Learners") are all sorcerers. Not all sorcerers are followers of Malkion (see Carmania in Glorantha: the Second Age for example).
The Middle Sea Empire is the Empire ruled by Jrustela and Seshnela, and it is essentially a mercantile power as it basically controls all of the seas after almost destroying the Waertagi. Malkionism is the Religeon of the Middle Sea Empire, and they are powerful Sorcerers.
"God Learnerism" is essentially a movement among the sorcerers of the Middle Sea Empire. God Learners have figured out how to enter the God Plane of Theistic/Divine beings and abuse, manipulate, or steal from the myths of the other cultures and gain great power from doing so. There are some factions within the Malkioni Church that consider the actions of God Learners as heretical - but since there is great power and wealth gained from the actions of the God Learners it is a very popular movement.
I hope this all helps as a quick and dirty primer to sorcery and God Learners. It is of course a simple overview, some God Learners see themselves as missionaries, benevolently learning of other cultures and hopefully coverting them to the true way, while others unabashedly steal from them for gain (they are just heathens after all) while others want to just wipe them out (Return to Rightness movement).
Note that according to Magic of Glorantha God Learners are 'better' sorcerers than non-God Learner sorcerers. They have some special spells and other spells have improved game effects (usually faster casting).
EDIT: Also, the Mostali are Sorcerers, and definately not God Learners.