First, a very important clue to note is that it tells us B's hidden letter is I.

From here, we can notice that there are a lot of words that ends with the letters ICH.
Could this be a common suffix? There are a lot of 3 letter suffixes though, for example: -ing, -ion, -ize, -ian, -ide, etc.

We can remove some possibilities by noticing the word GDICHICH.
It ends with two ICH's. There are not (or very rarely) any English words that ends with 'ionion', 'izeize', 'ianian' etc.

So, we can see that ICH have a high chance to be the prefix ING. Besides, it is also a really common suffix in English words.

Notice the word GAGINNING. There is only one 9-letter word that ends with GINNING, and that is the word BEGINNING.
So, we can deduce that GA is BE:

From the word FNNEEEEE, we can see that E is D.
Hence, the word will become FNNEEDED.
And now we can know that F means U. This will finally become the word UNNEEDED.

For BDINGING, there are two possibilities for the letter D: It can be either R for BRINGING or L for BLINGING.
But the possibility for D being L is low, due to the word DEBIFNEING will now start with LEB.
As additional evidence, the only 10-letter word that starts with LEB is LEBENSRAUM, which does not fit with our description.
So, D is R.

Finally, we can just see that I is O.
The hidden word will now be REBOUNDING, and that is our final answer.