So, here is what I found about the Goalkeeper-Sweeper so far.
Goalkeeper-Sweeper is a nickname refers to the usage of goalkeeper as sweeper; Or in other words when opponent plays a clever through ball behind the defensive line the goalkeeper will exit his beloved sticks and chase the ball like a maniac. After few checks watching the full match mode in my own game with an Icelandic First Division team I discovered the best settings for employing GS is when using a high defensive line or normal with slight push up from the values of 12-17, now the space between the defensive line and the goalkeeper will increase, attracting balls in it which will be picked up cheaply and gain possession by the goalkeeper.
When playing against quality teams with 'pacey' forwards it's better using a deep defensive line and a sweeper goalkeeper, so in the race to the wanted ball not only the defender and the opponent striker will be, but also your goalkeeper in the opposite direction. Keep in mind that with these settings you'll be most vulnerable to outside of the boxes shots: The goalkeeper is out of the goal slightly and the defensive line invites pressure so a defensive midfielder is probably the best option to prevent deflection-shots goals in that case.
The normal settings of GS are known as high closing down between the values of 15-18, mentality could be set to defensive or attacking as by my tests it basically tells whom he most likely sent a long pass.
Why would you use GS? To tell the truth, using the default settings is probably the safest option, but when using the goalkeeper as sweeper you enhance his contribution to the game, you gain possession cheaply, and have the option to start a counter attack.
What goalkeeper' attributes needed for this? My Icelandic goalkeeper is one of the worst goalkeepers I've ever seen and he was able to do the GS role pretty well, but in order to maximize the effect it's better to choose rather agile, quick goalkeeper (in terms of goalkeepers quick means pace and accelerations 10+ with exception of Reina from Liverpool), with high rushing out, communication, decisions, when one-on-ones is better than the shot stopping attributes, and an option of counter attacks will be kicking/throwing (depends on your distribution value).
Through balls into channels are less likely to be picked up by goalkeeper tough, so mind that when playing against strong wings-play teams.
How can I tell if I employed GS well? In the pitch the goalkeeper movement when the ball is in opponent possession will be more flowing, especially in one-on-ones situation since GW tend to exit their goal quick. In the commentary I noticed one sentence is repeating when using GS: "'Name of Goalkeeper' gather up the loose ball", oh and keep in mind the goalkeepers with high eccentricity used as GW are likely to make foolish mistakes, so keep an eye on that.
Goalkeeper to Sweeper Discussion 
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