Hi Jannus,
You have few options to avoid the time out error.
Option 1:
Check the Internal tablelt_gty_vendormain value is not initial is the best way to avoid the time out error.
Option 2:
Check the KNUMH value is not initial in internal tablelt_gty_vendormain. If you have any line with no KNUMH value in internal table lt_gty_vendormain then the select query will try to fetch all the values from the table KONP.
Option 3:
If you still get the timeout error then add some more field in the where condition like Condition Type, Tax Code, Application which will improve the select query performance.
Let me know if you need any more help.
Regards,
Saravanan Madhappan