There's no such thing as a "registered gunsmith".
To be a gunsmith who charges for work, you need to have a Federal Firearms License (FFL), comply with all State or local business licenses and zoning requirements, and have a State tax number.
If you have a brain at all, you'll also have insurance so customers can't sue your socks off.
Here's the list of gunsmithing schools. The ones with the best reputation are Colorado School of Trades and Trinidad Jr. College.
NO mail order or internet training course will prepare you to be a professional gunsmith:
Colorado School of Trades
1575 Hoyt Street
Lakewood, CO 80215
Phone: 800-234-4594
Lassen Community College
P.O. Box 3000
Susanville, CA 96130
Phone: 530-257-4211
Modern Gun School
80 North Main Street, P.O. Box 846
St. Albans, VT 05478
Phone: 800-493-4114
Montgomery Community College
1011 Page Street
P.O. Box 787
Troy, NC 27371
Phone: 800-839-6222
Murray State College
One Murray Campus
Tishomingo, OK 73460
Phone: 580-371-2371
Penn Foster Career School
ICS-Intext
925 Oak Street
Scranton, PA 18515
Pennsylvania Gunsmith School
812 Ohio River Blvd.
Avalon
Pittsburgh, PA 15202
Phone: 412-766-1812
Piedmont Community College
1715 College Drive
P.O. Box 1197
Roxboro, NC 27573
Phone: 336-599-1181
Pine Technical Institute
900 4th Street
Pine City, MN 55063
Phone: 800-521-7463
Sonoran Desert Institute
10245 East Via Linda, Suite 102
Scottsdale, AZ 85258
Phone: 480-314-2102
Trinidad State Jr. College
600 Prospect
Trinidad, CO 81082
Phone: 800-621-8752
Yavapai College
1100 East Sheldon Street
Prescott, AZ 86301
Phone: 520-776-2150