The regulations for awarding titles are given in FIDE Title Regulations effective from 1 January 2022.
There are two routes to getting the GM title:
- norms and a minimum rating
- specific results in specific Championship events plus minimum rating
You correctly identify the minimum rating for a GM title via norms of 2500 however "specific results in specific Championship events" is also another route with a different minimum rating.
Here is what FIDE Title Regulations effective from 1 January 2022 has to say:
0.6 The Award of Titles
0.6.1 Titles may be awarded for specific results in specific Championship events, or are awarded on achieving a rating as laid down in these regulations. Such titles are confirmed by the QC Chairman on advice from the FIDE Office. They are then awarded by FIDE.
0.6.2 For a direct title to be awarded immediately an applicant has to have achieved at some time a minimum rating as follows:
IM 2200 WIM 2000
FM 2100 WFM 1900
CM 2000 WCM 1800
If an applicant is rated lower the title is awarded conditionally and will be awarded finally on request by the respective federation as soon as the minimum rating is achieved. Any player with a conditional title may take a lower title when they reach the required rating for that lower title.
0.6.3 Titles are also awarded based on applications with norms with a sufficient number of games. These titles shall be awarded by the FIDE Council on recommendation by the QC.
The titles and titles in which they may be gained via "specific results in specific Championship events" are described in Table for Direct Titles effective from 1 July 2017.
The events and positions which can be used to gain the GM title are as follows:
- World Cup - last 16
- Women's world championship - winning the title
- World over 65 championship - winning the title
- World over 50 championship - winning the title
- Continental championship - winning the title
Other events like World U20 and World U18 can give GM norms but not the title itself.
There is also a minimum rating requirement as noted earlier for these direct title awards. For GM that is 2300.
I recently came across a GM who has a peak rating 2495 and has obtained the GM title after completing 3 norms.
Is this some kind of exception or has the player completed another special requirement that I'm unaware of?
According to FIDE Title Regulations effective from 1 January 2022:
1.5.3 To have achieved at some time a rating as follows:
GM ≥ 2500
So, the 2500 minimum rating required is a one-time rating which can be after a game in the middle of a tournament. It does not have to be maintained through to the end of the rating period.