Question
i am doing a little school project research on tree
gall...what does the gall grow on the tree bark contains?
...on one reference medieval people used it to make iron
gall ink, is it the same?

Answer
An oak gall is a curious ball-like tumor, about the size of a small marble, that grows mainly on the leaves or twigs of oak trees. It is formed when the gall wasp lays its egg in the growing bud of the tree and a soft, pale green, apple-like sphere begins to form around the larva. The insect secretes a chemical that causes the cells in the leave or twig to grow and cover the eggs that are laid.

The galls mostly on oak trees contain a chemical called tannic acid which will stain.  They were ground and mixed with iron sulfate, gum arabic, and liquid (eg. rainwater, beer, or wine) to form an indelible black ink.

In the first century AD, Pliny the Elder described a basic chemical demonstration of the principle behind what would become the primary ink of the Middle Ages:  Papyrus soaked in tannin turns black upon contact with a solution of iron salt.

This was not used for actual ink at the time of Pliny, but "gallarum gummeosque commixtio" is already mentioned as an established writing ink around AD 420, in the  encyclopedia of the 7 liberal arts  by Martianus Capella.  However, the latest analyses have disproved dubious reports that this type of ink might have already been used on the famous Dead Sea Scrolls of Qumran (before AD 68).

Because of the secondary reaction discussed below, which makes it indelible, iron ink was once known as  encaustum  (Latin for "burned in", from the Greek enkauston, meaning painted in encaustic and fixed with heat).  This is the origin of the English word "ink" itself, and of its counterparts in a number of other languages:  encre (French), inchiostro (Italian), inkt (Dutch), inkoust (Czech)...

Indelible iron-gall ink is considered the most important ink in the development of Western civilization, up until the 20th century.  The best iron-gall inks were far superior to most modern inks, but the corrosiveness of some compositions (discussed below) regretfully led to the abandonment of all iron-gall inks in favor of more sophisticated recipes with lesser chemical aggressivity.

Iron-gall ink normally includes what is effectively a "Chinese ink" component, which provides both body (from gum arabic) and some initial coloring upon application of the ink.  Otherwise, the main pigmentation of iron-gall ink comes paradoxically from water-soluble ferrous chemicals with little color of their own:  When the ink dries in air, an oxidation occurs which turns these  ferrous  salts into insoluble  ferric  dark pigments.  In addition, iron-gall ink may react with parchment collagen or paper cellulose, in a totally indelible way.  Some poorly balanced iron-gall inks have even been observed to burn holes through paper.

It has been shown that an excess of ferrous salt in iron-gall ink leaves permanent traces of active soluble salts (not properly oxidized into inert pigments) which will catalyze the slow decomposition of cellulose, especially when acidity is present.  This corrosion is reduced with a proper balance in the composition of the ink.

Source:   http://biblaridion.blogspot.com/2005/06/ancient-gall-ink.html