The . in the guild level is actually a , so that's 12,519/13,790 and you need 1,271 (one thousand, two hundred and seventy one) more power points to level. At the end of each calculation period (the timer you see at the top of the screen when you are viewing a province) you guilds current power is added to your guilds level.
Lets say you hold 12 sectors all at power 6 that means at the end of the "day" your guild level would go from 12,519/13,790 to 12,591/13,790 (that is +72 points). If you take another sector the power points it's worth are added at the end of the day as a part of your guilds total power.
Lets say your guild takes a new sector that is worth 12, your guild at the end of the "day" will get 72+12=84 points per "day". So each day your guild will have 84 points added to it's "level" and over time level up. So day 1, 12,519/13790, day 2 (+72), 12,591/13790, day 3 (+84), 12,675/13790, day 4 (+84), 12,759/13790, and so on.
There are also modifiers (+5%-+15%) for being in the top 3 guilds of each province (and I think being in the top 3 guilds overall) but I'm unsure if the additional points worked out there are just for guild rankings or if your guild will actually gain those extra power points and level faster.
In the end the more power your guild can hold as a total the faster your guild will level (your guild will also hold a higher rank), you don't have to keep taking new sectors to gain power points. Other ways of gaining power may appear in future updates but currently the only way is to take sectors in GvG.