gon version: 6.4.0
Ruby version: 3.0.0
Rails version: 6.0.6

The error "ArgumentError - tried to create Proc object without a block" appears when <%= include_gon(init: true) %> is added on the main application.html.erb layout file.

Updating the dependency multi_json to version 1.15 appeared to resolve the issue. Looks like the issue was fixed in version 1.14.1
intridea/multi_json@26a94ab