Both "bathe" you in sound and vibrations that usually result in relaxation and better sleep. 

1. They differ in the instruments used.  My sound baths primarily use crystal bow and are complemented by chimes, gong, ocean drum, and more.  My gong baths primarily use gongs and are complimented by other instruments. 
2. They differ in their goal.  Sound baths aim to balane energy, relieve stress, and induce deep, light, and airy relaxation.  Gong baths aim to disrupt repetitive thought patterns and provide restorative effects for the nervous system.
3. They differ in your experience:  Sound baths have diverse frequencies which creates a varied and layered soundscape.  The tones are generally easier on participants.  Gong baths have deep full-body vibrations.  

Both use the same instruments.  They differ in time and experience.  My sound baths are 60 minutes and neurological sound baths are 30 minutes.  You lie down on a massage table or sit in a chair and experience the sound bath with eyes closed and stillness.  You participate in the neurological sound baths based on the participant.  Participants can play some of the complementary sound instruments, lie  down or sit, or draw/color.  Neurodivergent generally cannot tolerate the longer time or long stillness.  It will not be as relaxing as a sound bath; however, it will benefit the participant with reduced anxiety, reduced stress, better sleep, and such.

Reiki is not based on any religion and is not religious.  Mikao Usui developed Reiki in Japan. Reiki is a Japanese word.  Rei = "univeral" and Ki = "life energy".  

It is a non-invasive complementary therapy which doesn't cure anything but instead calms the nervous system, alleviates anxiety, and releases tension.  I believe it resotres a natural energy balane to facilitate the body's own healing response.