This is mostly inspired from the previous answer, which had some flaws.
select max(next_x) from
(
select
count(case when y='B' then 1 end) over (order by x) correct,
count(case when y='G' then 1 end) over (order by x) wrong,
lead(x) over (order by x) next_x
from table_name
)
where correct/(correct + wrong) > 0.99
Sample data:
create table table_name(x number, y varchar2(1));
insert into table_name
select 2, 'B' from dual union all
select 3, 'B' from dual union all
select 3, 'B' from dual union all
select 4, 'B' from dual union all
select 5, 'B' from dual union all
select 5, 'B' from dual union all
select 5, 'B' from dual union all
select 6, 'G' from dual union all
select 7, 'B' from dual union all
select 7, 'B' from dual union all
select 7, 'B' from dual union all
select 8, 'B' from dual union all
select 8, 'B' from dual union all
select 8, 'B' from dual union all
select 12, 'G' from dual union all
select 12, 'G' from dual union all
select 12, 'G' from dual union all
select 12, 'G' from dual union all
select 12, 'G' from dual union all
select 12, 'G' from dual union all
select 12, 'G' from dual union all
select 12, 'G' from dual union all
select 13, 'G' from dual union all
select 13, 'G' from dual union all
select 13, 'B' from dual union all
select 13, 'G' from dual union all
select 13, 'G' from dual union all
select 13, 'G' from dual union all
select 13, 'G' from dual union all
select 13, 'G' from dual union all
select 14, 'B' from dual union all
select 14, 'G' from dual union all
select 14, 'G' from dual;