The big difference between GSAK and PGC (apart from being able to view everyone's stats easily in PGC and being able to view only one or two others with difficulty in GSAK), is that in GSAK you have control over the database, whereas in PGC you do not.
In GSAK you can make queries to cut and dice your statistics or find targets any way you want and then present that data any way you want. There are no limits. You have the data, you can make the queries (or use other people's macros, which is essentially the same thing). In PGC you do not have access to the query building tool, all you can do is use the preset queries (and their options) as already provided. Luckily, PGC has a huge range of preset queries, which easily answer the vast majority of requirements.
Learning how to build a GSAK query will not really help with interacting with PGC, simply because the flexibility is not there.
I recommend just getting familiar with PGC. Try out every query, on your own data and on your friends. Learn what they do. Make sure you try the filter options as well!