Ok! So it sounds like you want to have a few items grouped together for each of the four separate bios. Each of the four groups will contain the bio itself, a few questions in MediaLab, a GNAT, and a debriefing.

When starting this task, the easiest way to proceed would be to create the questionnaires needed for each bio, the questionnaires needed for each batch of questions, the GNATs required, and the debriefing (as a separate questionnaire).

You would want to list each group of items one after the other within your questionnaire. For example:
1. Bio_1
2. Questions_1
3. GNAT_1

4. Bio_2
5. Questions_2
6. GNAT_2

7. Bio_3
8. Questions_3
9. GNAT_3

10. Bio_4
11. Questions_4
12. GNAT_4

13. Debriefing

Once you've created this setup, you'll want to assign a BGR value of 1 to items 1, 2, and 3; a BRG value of 2 to items 4, 5, and 6; a BGR value of 3 to items 7, 8, and 9; and a BGR value of 4 to items 10, 11, and 12. The debriefing would require a BGR value of 0. If the GNAT is completed through DirectRT, you would need another item per group after each GNAT that consists of a questionnaire with only one item. Upon the items completion, you could denote that a skip is made to the item name within the debriefing questionnaire.

For example, in a similar thread, a user asked how they could simply skip to an item similar to the debriefing item that you are requiring your users are skipped to. Here's what Blair noted:
At the end of the experiment, you could add a single item questionnaire. You could have the last item in each of the other questionnaires skip to THAT item when they are done. That way your participants see only one of the five questionnaires. The others are all skipped over. Does that make sense?
MediaLab has the ability to skip to items within other questionnaires. Simply place the variable name of the questionnaire item you want to skip to within the first SkipTo column that is presented to you within the MediaLab experiment editor.

Does this make a bit more sense? I can gladly create a mock-up example for you. Are the GNATs going to be completed through DirectRT?