Here's what George F. Black has to say about the MacGregors:
MACGREGOR, MACGREIGOR, MACGRIGOR. Gaelic MacGriogair, ‘son of Gregory.’ Gregory, the name of several early popes, was a favorite name in the middle ages. The name of Gino or Gyric, fourth in succession from Kenneth I, was elaborated by Fordun (Chron., IV, 16) into Gregorius, and is the ‘Gregory the Great’ of feudal fabu]ists. He is the legendary ancestor of the Macgregors. See under GRIRIC.
"The Macgregors," says R. H. Bruce Lockhart, "had the redeeming merit of picturesqueness, and for that reason they occupy a larger place in Scottish literature than any other Highland clan" (A Son of Scotland, 1938, p. 131). On account of the many lawless deeds justly or unjustly laid to their charge, the name of Macgregor was proscribed in 1603 by Act of Parliament. Many of the clan in 1606 in obedience to an Act of the Privy Council renounced the name and adopted the surnames of Stewart, Grant, Dougall, Ramsay, and Cunninghame (Coil., p. 130, 131, 135).
The Rev. James MacGregor, one of the founders and first pastor of Londonderry, New Hampshire, was of Scottish origin. The proscription of the name was rescinded in 1661 but revived in 1693, and not finally abolished until the year 1784, after it had been for several years in practical desuetude. Though the name is quite widespread today it is doubtless not borne by all who are entitled to it. A list of those who were fined for resetting outlawed members of the clan in 1613 is printed in RPC., xiv, p. 629—663. In New York state in 1689 we have mention in a Dutch document of "Macgrigerie uit Schotlandt," and in 1697 of one Patrick Macgrigari. M’Gregare 1500, M’Gregur 1600, McGreigor 1682, M’Grigar 1711, Griger McGriger 1696, McGrigour 1586, Makriggour 1600. See also under GREGOR.
An act anent the Clan Gregor passed in 1617 refers to the earlier act of April 1603 whereby "It wes ordanit that the name of mc gregoure sulde be altogidder aboisched And that the haill persounes of thatt clan suld renunce thair name and tak thame sum other name And that They nor nane of thair posteritie suld call thame selffis gregor or mc gregoure thairefter under payne of deade." The act of 1617 referring to the many children of the clan now "ryiseing up and approcheing to the yearis of maiorite approves of the earlier act and another of 1613 referring
to the clan, further declares and ordains "that gif onie persoun or persounes of the said Clane who lies alreddie renunceit thair names or heirefter sail renunce and cheinge thair names Or gif onye of thair bairnes and posteritie sail at ony time heirefter Assume or tak to thame selffis the name of gregoure or Mc gregoure ... That everie sick persoun or persones assomeing and taking to thame selffis the said name.., sall Incurre the payne of Deathe qlk payne salbe execute upoun thame vithout fauoure" (APS., xv, p. 550—551).