Prior Group ID Gap only applies to Objects, not Tracks.
Prior Group ID Gap (Property Type 0x3C) is a variable length integer containing the number of Groups prior to the current Group that do not and will never exist. For example, if the Original Publisher is publishing an Object in Group 7 and knows it will never publish any Objects in Group 8 or Group 9, it can include Prior Group ID Gap = 2 in any number of Objects in Group 10, as it sees fit. A Track is considered malformed (see Section 2.4.2) if any of the following conditions are detected:
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An Object contains more than one instance of Prior Group ID Gap.
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A Group contains more than one Object with different values for Prior Group ID Gap.
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An Object has a Prior Group ID Gap larger than the Group ID.
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An endpoint receives an Object with a Prior Group ID Gap covering an Object it previously received.
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An endpoint receives an Object with a Group ID within a previously communicated gap.
Use of this property is optional, as publishers might not know the prior gap size, or there may not be a gap. If Prior Group ID Gap is not present, the receiver cannot infer any information about the existence of prior groups (see Section 2.3.1).
This property can be added by the Original Publisher, but MUST NOT be added by relays. This property MAY be removed by a relay when the object in question is served via FETCH, and the gap that the extension communicates is already communicated implicitly in the FETCH response; it MUST NOT be modified or removed otherwise.
An Object MUST NOT contain more than one instance of this property.