Gelatine is Kosher - AchiEzer
Gelatine and collagen are different names for the same product, see. These days it is usually extracted from skin scraps not suitable for the leather industry although collagen is also found in bones and flesh. Halacha deems it to be a new entity, denatured, disconnected from its origin since during processing it is rendered entirely unsuitable as a food.
The great Gaon, the Achiezer, HaRav Reb Chaim Ozer Grodzeski, the Rav of Vilna, in Teshuvah 33 [original], explains that gelatine is Kosher. It is no different to cheese which is Kosher and indeed may be made in the first instance, with the rennet from a non-Kosher beast [ShA YD 87:9].
The cheese is Kosher because the stomach from which the rennet is extracted, is Kosher.
Can the stomach of a non-Kosher beast become Kosher?
Yes, once thoroughly dried out, it is no longer deemed to be meat; in fact it is no longer deemed to be food. In the parlance of Halacha, it is wood or stone. Stones are not a food, so there is no need to explain why a stone is Kosher.
So, gelatine and rennet the enzymes produced by glands in the stomach are Kosher - they may have once been a food but are no longer.
Reb Chaim Ozer dismisses the argument that the skin [or bones] from which the collagen is extracted, having been cooked with meat, will require Kashering. He explains that they are in fact more than Kashered by the extraction process itself.
He supports this from:
Untruths are sometimes honourably motivated. And so we find honourable people who refuse to acknowledge the Pesak that Gelatine is Kosher. This ruling is most famously known from Reb Ch Ozer Grodzenski, the famous Rav of Vilna and international Posek. His ruling was not disputed during his lifetime nor for many decades after his passing.
It is false and misleading to claim, "permitting animal gelatin is based on an almost universally rejected Halachic leniency".
To be sure, there are Poskim and Kosher orgs that will not certify gelatine, but this appears to be based more upon Haredi community expectations than compelling Halachic argument. In fact, Rav Tzvi Pesach Frank, Rav Ovadia Yosef and Rav Eliezer Waldenberg, permit it and virtually every gelatin manufacturer in the US manufactures "Kosher" gelatin!
The argument that these days gelatine must be non-Kosher since it is processed from skins that are edible, is a sham. Halacha has long recognised that some skins are edible [see RaMBaM MaAsuros 4:20&21] and besides the foundation argument permitting gelatine is that the stomach of a non-Kosher animal, which is certainly edible, will ceases being a non-Kosher food when it is no longer deemed to be a food. And so it is with gelitane.
In the interest of balance, we recognise that this matter is disputed by some, however, even the Machmirim, the Poskim who prohibit gelatine, concede that beef gelatine as opposed to fish gelatine is not meat at all or not truly meat. HaRav Moshe Feinstein explains that although Kosher gelatine must be extracted from Kosher hides, it is nevertheless Pareve.
The Beis Din of the Agudas Yisroel in Yerushalayim also ruled that such material would be Pareve.
HaRav Aharon Kotler, while ruling that it is not Pareve, nevertheless concedes it may be cooked with dairy where it is less than 1/60 of the product.
The following is published by a Kosher org but it is false and misleading - Although animal bones and hides are considered inedible and “Kosher” even if from a non-kosher or non-slaughtered animal (Rambam MaAcholos Asuros 4:18), and are only forbidden MiDeRabannan, those skins which are edible, like pig skin, are not kosher. Gelatine made from such skins, could not be easily defended as being kosher .