You can lookup the name of the underlying sequence by checking the data_default in *_tab_cols. Then find the current value for this and add the increment value.

create table t (
  x int generated as identity
);

insert into t values (default);

select * from t;

X  
1  

select column_name, data_default from user_tab_cols
where  table_name = 'T';

COLUMN_NAME  DATA_DEFAULT                     
X            "CHRIS"."ISEQ$$_197255".nextval 

select ISEQ$$_197255.currval + increment_by 
from   user_sequences
where  sequence_name = 'ISEQ$$_197255';

ISEQ$$_197255.CURRVAL+INCREMENT_BY  
2

insert into t values (default);

select * from t;

X  
1  
2