To be certain, you should:
* Consult your owner's manual: This is the definitive source.
* Look closely under the hood: Systematically check both the fender apron and the firewall area. The fuse box is usually a fairly noticeable rectangular box.
* Check online resources: Searching for "1977 Pontiac Grand Prix fuse box location" along with images might reveal pictures showing its location on a similar model.
Keep in mind that some models might have an additional fuse block, perhaps in the interior, for smaller circuits. However, the main fuse box is always under the hood.