I just entered the name Gun** on SP, Caithness, no parish, father Robert and the names you mention, results below, though not necessarily the exact date for Donald.
LATHERON BAPTISMS FATHER ROBERT GUN
DOND 2/05/1757, JOHN 30/08/1759, David & Elizabeth 5/07/1764.
There is a total of 11 baptisms father Robert in Latheron 1755 – 1789, the last being Alexander.
There are a few GUNN baptisms at Thurso, Watten and one at Halkirk, these all name both parents.
I cannot see a marriage in Scotland on SP for Robert 1745 - 1755.
Colin