Hello!

Is it possible to put Gandalf in my Gondor army?
Possibily aragorn too, I have one where he is on foot (this one: http://1.bp.blogspot.com/-YDp__rcxS9Y/U ... 1600/1.JPG)

You can ally Gandalf, either Grey or white, but he cannot lead any troop. Gandalf is either part of the Fellowship List or the White Council List.

As For Aragorn, there are 3 version of him: Isildur's Heir (Arnor), Strider (Fellowship) and King Elessar. King Elessar is a hero of Gondor, which means that he can lead a warband of gondor troops. However, King Elessar comes with Anduril and Heavy Armor and it's quite expensive point-wise
If you want to play Aragorn, as a hero of Gondor, you can use your Helm's Deep Version as a Proxy, however, you have to remember that Elessar is 80 points more expensive than that when you make your army list.

If you want to play with Aragorn, but not the King Elessar one, you have to ally. Gandalf and Aragorn/Strider are both part of the Fellowship, so you can put them together in a single allied warband if you want.