I'm afraid that you have flumuxed me. I still believe my explanation of GMP is correct - and it seems to be supported by this.
http://www.sharingpensions.co.uk/glossary14.ht m#text5
(scroll down a bit).
But since a male currently needs 44 qualifying years to get a full basic state pension state, I don't understand how you are getting the full basic state pension as you were contracted out for some of your working life. (Unless the contracted out period was very short and you started work at say 16 - so still managed to get the full 44 years in).
My comment about the 3% cap not being an issue until now was made because, with price inflation under 3%, private sector schemes have been able to outstrip price inflation if trustees have chosen to give benefits at up to 3% annual inflation. You have only just retired, so have not benefited, and worse still, inflation has just taken off and the scheme is capped. I can only suggest you address this question to your pension trustees.
This is entirely a guess, but maybe Government originally prevented private schemes inflating their GMPs by more than 3% to prevent any temptation to provide benefits for current retirees in preference to future generations of retirees. It could also be seen as an anti-inflationary measure.