There is a systematic mention of Ghar in headings of Sri Guru Granth Sahib. Once mentioned, the heading of any Ghar is applicable to all the following sabads till there is a change in the Ghar of sabad or chhant which is duly depicted in the heading.
The mention of Ghar in heading corresponds to the change in Mahalla under each raag. Ghar is often mentioned in the heading with every change in Mahalla. The only reason for non-mention of Ghar with change in Mahalla is the continuity of the same Ghar between Mahallas.
There is always a mention of Ghar in heading, whenever, there is a sabad/chhant in any Ghar except Ghar 1 and Ghar 2. The sabads/chhants following thereafter, in the same Ghar, do not bear Ghar in their headings, since all the following sabads/chhants, thereafter, are deemed to be in the same Ghar.
However, Ghar 1 and Ghar 2 may not necessarily be mentioned in heading, even on first sabad.
We can discuss one of the raag to make things clearer. Let us take Raag Gond at SGGS 859. In the beginning of Raag Gond, there are sabads of Mahalla 4 and Ghar 1 is mentioned in the heading. There are six sabads of Mahalla 4. Whereas, the heading of Ghar 1 is only on first sabad, and following five sabads are deemed (and established) to be in Ghar 1. Thereafter, while still Ghar 1 is continuing, but there is change in Mahalla. Now there are sabads by Mahalla 5 (SGGS 862). Therefore, there is repeat of heading of Ghar 1. There are 22 sabads of Mahalla 5. But all sabads are not in Ghar 1. In fact, after only two sabads, there is change in Ghar from Ghar 1 to Ghar 2. And this change is mentioned in the heading. Now, the remaining 20 sabads of Mahalla 5 are deemed (and established to be) in Ghar 2, therefore, there is no further mention of Ghar 2 on these sabads.
After sabads of Mahalla 5, there is start of Astpadi at SGGS 869. Although Astpadi is also in Ghar 2, but there is a mention of Ghar 2 again at the start of Astpadia.
After Astpadi, there are sabads of Kabir ji (SGGS 870). But now sabads by Kabir ji are in Ghar 1, therefore, Ghar 1 is mentioned on the first sabad. But all sabads of Kabir ji are not in Ghar 1. After three sabads, there is change in Ghar 1 to Ghar 2 and it is mentioned in the heading. There are seven sabads thereafter, but there is no more heading since all the following sabads are deemed (and established to be) in Ghar 2. After sabads of Kabir ji, there are sabads by Namdev ji. There are a total of seven sabads by Namdev ji. There is a heading Ghar 1 on first sabad and Ghar 2 on fifth sabad, thereby implying that, out of seven sabads of Namdev ji, first four are in Ghar 1 and last three are in Ghar 2. Now, although the last sabad of Namdev ji is in Ghar 2, yet when the sabads by Ravidas ji start (SGGS 875), Ghar 2 is mentioned again once for the two sabads by Ravidas ji wherein the second sabad is deemed (and established to be in ) Ghar 2.
Whenever, Ghar of a sabad is changed to any Ghar, (other than Ghar 1 or Ghar 2), the Ghar is always mentioned on such sabads, though Ghar would not be mentioned on all following sabads, so long they continue to be in the same Ghar. For example, at SGGS 715, in Raag Todi, Ghar changes from Ghar 2 to Ghar 3. Ghar 3 is duly mentioned on the first sabad at SGGS 715. It is followed by another sabad which is also in Ghar 3 but there is no mention of Ghar 3 in the heading, though the sabad is in Ghar 3. Then, Ghar changes from Ghar 3 to Ghar 4, and is duly mentioned at SGGS 715. Again there are two sabads in Ghar 4. Therefore, Ghar 4 is not mentioned on the second sabad. The Ghar again changes to Ghar 5. And Ghar 5 it is mentioned at SGGS 715. And then there are 12 sabads in Ghar 5 till SGGS 718. There is no mention of Ghar 5 on any of these sabads, since the mention of Ghar 5 has already been made at SGGS 715 and it is applicable to all the following sabads.
At SGGS 841, there is Mahalla 3 Vaar Sat in Ghar 10. By SGGS 843, chhants in Mahalla 1 begin and Ghar 10 changes to Ghar 1. There is, however, no mention of Ghar at the change of Mahalla from Mahalla 3 to Mahalla 1, along with change in Ghar from Ghar 10 to Ghar 1, at SGGS 843. Thereafter, at SGGS 844, there are more chhants in which though Ghar remains same, there is change in Mahalla from Mahalla 1 to Mahalla 4, yet there is no mention of Ghar because there is continuation of Ghar 1. Later, at SGGS 845, there is again change in Mahalla from Mahalla 4 to Mahalla 5, yet there is again no mention of Ghar because of further continuation of Ghar 1. Further, at SGGS 855, chhants are followed by sabads by Kabir jio yet there is no mention of Ghar in heading since Ghar has changed from Ghar 1 to Ghar 2. Further still, at SGGS 857, there is sabad by Namdev ji, followed by two sabads by Ravidas and one sabad by Sadhna, yet, there is no mention of Ghar in the heading because Ghar of all sabads continue to be in Ghar 2.
The Ghar is not compulsorily mentioned with every change in Mahalla, if the Ghar of the sabads in either Mahallas is same. An example of no mention of Ghar with change in Mahalla, as is at SGGS 718, wherein there is no mention of Ghar while there is change of Mahalla 5 to Mahalla 9.
Thus, Ghar of a sabad is always mentioned in the heading in Sri Guru Granth Sahib in case it is in any type of Ghar other than Ghar 1 and Ghar 2.
Whenever there is no mention of Ghar in the heading of a sabad, there are two possibilities. Either there is mention of Ghar on a preceding sabad that is applicable to the sabad or the sabad belongs to Ghar 1 or Ghar