You can assign two ROW_NUMBERs based on a random sort and join on them:

select
   t.id,
   g.id as gebruikerid
from
 (
   select id,
      (row_number() over (order by newid())
    % (select count(*) from gebruiker))+1 as rn -- number from 1 to n 
   from topic
 ) as t 
join 
 ( select id,
     row_number() over (order by newid()) as rn -- sequential number from 1 to n
   from gebruiker
 ) as g
  on t.rn = g.rn 
order by t.id

See fiddle

You can assign two ROW_NUMBERs based on a random sort and join on them:

select
   t.id,
   g.id as gebruikerid
from
 (
   select id,
      (row_number() over (order by newid())
    % (select count(*) from gebruiker))+1 as rn -- number from 1 to n 
   from topic
 ) as t 
join 
 ( select id,
     row_number() over (order by newid()) as rn -- sequential number from 1 to n
   from gebruiker
 ) as g
  on t.rn = g.rn 
order by t.id

See fiddle

EDIT:

You can use the result of this SELECT to update the Topic table:

update t
set gebruikerid = dt.gebruikerid
from topic as t
join
 (
   select
      t.id,
      g.id as gebruikerid
   from
    (
      select id,
         (row_number() over (order by newid()) % (select count(*) from gebruiker))+1 as rn
      from topic
    ) as t 
   join 
    ( select id,
         row_number() over (order by newid()) as rn
      from gebruiker
    ) as g
   on t.rn = g.rn 
 ) as dt
on t.id = dt.id;

fiddle