The GXP novels are essentially the producer's cut of GXP.
Due to the nature of anime series being only 26 episodes, Kajishima felt some material had to be cut, and then his director didn't help the situation he glossed over a lot and overemphasized the humor ignoring the plot. So when it was done he felt the need to do it again properly and now it's the version of the story the story uses.
When you read the novels you will notice that things which show up in the anime very slightly are now fully developed and explained some were things you never even noticed before, but their in the books.
The entire first part (1-14) has been translated and proofread.
The OVA 4 Novel should be read between novels 4 and 5.
link: PDFs and TTS audiobooks
EVERYTHING UNDER HERE IS OLD AND DEFUNCT COMPARED TO ABOVE
EP
1-4
EP
5
EP
6-7
EP
7-8
Cut
EP 1
EP9-10
Cut
EP 2
EP 11-12
EP
12-13
|GXP 1|
|GXP 2|
|GXP 3|
|GXP 4|
|GXP 5|
|GXP 6|
|GXP 7|
EP
14-15
EP
16-17
EP
18-19
EP
20
EP
21-23
EP
23-24
EP
25-26
|GXP 8|
|GXP 9|
|GXP 10|
|GXP 11|
|GXP 12|
|GXP 13|
|GXP 14|
Renza
arc:
|GXP 15|
|GXP 16|
|GXP 17|
----------------------
What's left:
| GXP 8 | GXP 9 | GXP 10 | GXP 11 | GXP 12| GXP 13 |GXP 17|