Your guild level is determined by taking the average (rounded to the closest integer) of your 14 primary skills. Only the real, useful skills at the end of the tree, like fa.ri.de.se or fa.po, NOT fa.ri.of. In other words, you'll gain a GL every 14 levels in a skill. It is a common misunderstanding that your level in Faith is related to your GL. The difference is this: your level in Faith is determined by averaging your levels in fa.ri, fa.it and fa.po, where fa.ri is determined by averaging your levels in fa.ri.of, fa.ri.de, fa.ri.cu, fa.ri.mi and fa.ri.sp, and so on. Therefore, fa.po has much more impact on your Faith level than any other subskill. This causes the difference between your GL and your Faith level.
The bottom line of 'skills primaries' will show how many primary skill levels are left until reaching the next guild level.