I would suggest taking a week long course. Something like LFI I, that will run closer to $800 but it is offer in 2 segments over weekend to make it easier for us average people to fit in budget and schedule.
Other side of that idea is you will need 500-1000 rounds of practice ammo for the gun for a class like that.
With ammo prices setting aside basic amount of ammo to practice will run close to that $400-500 mark.
So I would suggest buying one or two good holsters, getting 6-8 speedloaders at least 500 rounds of ammo, thousand rounds more than twice as good. Then take a class with Tom Givens
http://www.rangemaster.com/
Farnam
http://www.defense-training.com/
Ayoob
http://massadayoobgroup.com/?page_id=121
or something similar.
Note Farnam and Ayoob travel a lot for classes so you don't have to travel far for one of their classes usually.
After that level of training, plus
REGULAR practice, I would suggest getting a second handgun. One that would work as pocket or BUG [Back Up Gun=2nd gun carried in case first one doesn't work, runs dry, or you need to arm someone that is qualified to use hangun in emergency but doesn't have one].
Since you have GP-100 and seem to like it I would suggest SP-101 or Glock 26 as two guns to look at for 2nd handgun.
After you have 2nd handgun and proper holsters & etc for it, then I would get long gun as 3rd gun.
12 gauge is good. But I think for most people it is way to much of a good thing for basic home defense.
IMO the 12 gauge is sort of like a PH's stopping rifle. It is big and heavy and kicks way to hard. BTW recoil wise full power 12 gauge is on par with 375 H&H full power loads.
It is best choice in only a very small nich. It is a fair choice for many self defense situations. And frequently a bad choice.
The problem with the 12 gauge is not a lack of power. The problem is that it has way to much power and size!
If your going to use a pump 12 for home defense stick to low recoil buck and slugs.
The 20 gauge for shotgun or 5.56/223 for carbine are much superior choices for home defense.
The 20 gauge has plenty of power, it is about equal to the 454 Casul, but is much easier to move with indoors because it is smaller than a 12. And the lower recoil allows one to engage multiple targets quicker than the 12.
I used to have two 12 gauge 11-87's plus an AR. Talked my girlfriend into trying 20 gauge Youth 11-87, she didn't like the AR, she really loved the 20 gauge.
After she got the 20 I compared it to my 12's, the weight and size difference of the 20 made it much easier and faster to move with than the 12. On the range the 20 is a lot faster to shoot.
I have traded one of my 12's off already, and once I get a hunting caliber upper for my AR the last 12 will probably go.
Another plus IMO with the remington is their is no dimple in the 20's magtube so adding mag extension was easy.
Now if you already owned a 12 that you used for skeet or hunting that is a different story. You would have lots of skill built up from practice with it already. And you would be saving money over switching to a whole new system.
I would still suggest using low recoil ammo unless your using a semi auto.