Use both...a leash for the GL and one for the flat collar. Hold each leash in a seaparate hand. Only use the GL when absolutely necessary. Hopefully, you've been training him all along to walk nicely on a loose leash without a GL on....in easy-to-succeed areas. (no to low distractions) Then just gradually wean him off the GL so that you don't need to use it at all. Be sure and use plenty of high value, frequent reinforcers for the behavior you like and set him up for success by walking in areas that are easy for him...at first.