⚠ Disclaimer: I am still learning about the goshin-ho and thus the below is best seen as a work in progress… Anyone with a more complete answer is welcome to post it. Any errors, confusion, and misrepresentations are mine alone.
What is it?
The term goshin-ho means "methods of self defence" and as such is not a kata.
There is a set of (57) techniques showing basic Aikido skills of economy of movement, good posture, and application of power via movement that form the
4th to 7th Shodokan syllabus.
Each syllabus demonstrates different themes and skills:
4th dan explores kuzushi and applications techniques from go no sen timing points. Many of the techniques are variation of the nage no kata with atemi and kansetsu waza finishes.
5th dan explores the theme of leading an opponent into a weaker posture before executing technique.
6th dan explores a variate of techniques in which the jacket is grabbed in different ways.
7th dan revisits the 6th dan syllabus and explores the use of kegatana, datsu roku, and kaiten.
Note that 7th dan is the last examinable rank of the Shodokan system.
Where does it fit?
Tomiki added it to allow higher grades to go back to basics and practice them some more so they could understand them better.
This makes sense in the grand scheme of dan grades: At shodan (1st), you know the basics. At nidan (2nd), you can do the basics. At sandan (3rd), you understand the basics. This is the "obey" stage of shu-ha-ri (守破離). At yondan (4th), godan (5th), and rokudan (6th) you put the basics into practice and start to deviate/digress from your teacher: This is what the goshin-ho helps you achieve.
Much later on, nanadan (7th) and above, you should leave your teacher and make your own way. This is the reason why 8th dan is seen as the traditional Meyko-kaiden: your teacher has nothing left to teach you.
Most of the above was gleamed from Scott Allbright either from his Aikido and Randori, reconciliation of two opposing forces book or from personal conversations.