There isn't one single definitive Gaelic spelling for "Peter." Gaelic encompasses several different languages, and the spelling would vary depending on which one you're using:
* Scottish Gaelic: The most common spelling is Peadar.
* Irish Gaelic: The most common spelling is Peadar as well. Variations might exist depending on dialect.
So while both use "Peadar," it's crucial to specify which Gaelic you mean.