SQL Server 2016

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create table tab( str varchar(20) )
insert into tab
values('ABCD'),
      ('hfdff'),
      ('uiyuiyoy'),
      ('uytitito'),
      ('piooioi'),
      ('GXX jhjkhh'),
      ('GXX uyyto'),
      ('GXX upupu'),
      ('tytuyur'),
      ('zvsfsgsg')
      
10 rows affected

select *
  from tab
 order by case when substring(str,1,3) = 'GXX' then 1 else 0 end, str
           
str

ABCD

hfdff

piooioi

tytuyur

uiyuiyoy

uytitito

zvsfsgsg

GXX jhjkhh

GXX upupu

GXX uyyto

select *
  from tab
  order by case when substring(str,1,3) = 'GXX' then 1 else 0 end,
           case when substring(str,5,1) = 'u' then str else 'z' end desc
            
str

ABCD

hfdff

uiyuiyoy

uytitito

piooioi

tytuyur

zvsfsgsg

GXX jhjkhh

GXX uyyto

GXX upupu