Managed to retrieve the GUID as below.
DATA: lr_gdc TYPE REF TO if_crm_ui_data_context,
lr_current_bt TYPE REF TO if_bol_bo_property_access.
* Create an instance of the data context.
lr_gdc ?= cl_crm_ui_data_context_srv=>get_instance( ).
IF lr_gdc IS BOUND.
* Get the Business Transaction entity
lr_current_bt ?= lr_gdc->get_entity( if_iccmp_global_data_cont_con=>gdc_currentbt ). '
* Get the Business Transaction GUID
ls_guid = lr_current_bt->get_property_as_string( 'CRM_GUID' ).
ENDIF.