this rule apply for every najasat? For e.g. My paint got najis from drainage (gutters and all) so it is permissable to take ghusl in the morning?

There are only 4 wajib Ghusls for men
1- Ghusl e Janabat
2- Ghusl e Maiyyet
3-Ghusl Mass e Maiyyet
4- Ghusl due to Nazr/Qasam/Ahad
Other than that others najasat or najis things do not required Ghusl.