content_migration.yml

id: the_content
label: Your Content Migration
migration_group: whatever_you_call_this_group
source:
  plugin: your_content_source

destination:
  plugin: entity:your_content_type

process:
  type:
    plugin: default_value
    default_value: your_content_type

  # whatever you want to map on content

group_content.yml

id: group_content
label: Your Group Content Migration
migration_group: whatever_you_call_this_group
source:
  plugin: your_content_source

destination:
  plugin: entity:group_content

process:

  gid:
    source: mt_id
    plugin: migration
    migration: the_content
    
  entity_id:
    source: the_source_content_id
    plugin: migration
    migration: the_content

migration_dependencies:
  required:
    - the_content
    - the_groups

group_migration.yml

id: the_groups
label: Your Group Migration
migration_group: whatever_you_call_this_group
source:
  plugin: a_source_for_groups

destination:
  plugin: entity:group

process:
  type:
    plugin: default_value
    default_value: your_group_type

  # whatever else you need to map...