Achiezer (3:33:5): Gelatine is made from dry bones that have been soaked in lime and other chemicals, the extract is then dried and processed into a fine powder. This powder is used to thicken foods.
This is Kosher based upon the same reasoning that the Rema applies when he Paskens that the stomach flesh of non-Kosher beasts is Kosher when it is salted and dried, and may be used in the first instance to make Kosher cheese. The Shach observes that this is equally true about all internal organs (and this would include the bones skin and cartilage). [implying the internal organs have a lesser status whereas the muscle the 'regular meat' will never be demoted from its status as meat]
Many Acharonim similarly permit blood and worms that became as hard as wood.
There is further proof from RaMBaM's Pesak regarding a corpse which has deteriorated; from the Yerushalmi in Orlah Ch 3; from the Rama YD 155 regarding incinerated insects; Gelatine will therefore certainly be Kosher since it is derived from bones which are not even Assur as Neveilah or Treifah, as Paskened by RaMBaM.
Furthermore, in this case there is no concern at all since bones are not Assur, as we see from YD 99, bones are calculated as part of the Hetter when in a mixture of Kosher and non-Kosher. And even soft bones which the Shach deems to be Assur become Muttar when all flesh is removed and they are dried.
There is no concern that the bones absorb Neveilah when they are cooked with the Neveilah meat which is still attached to them because any flavour has been completely neutralised via the chemicals used to process the bones, as we see from the Rashba whose position is Paskened by ShA OCh 447, regarding Nosein TaAm LifGam during Pesach which is Assur unless the flavour is completely neutralised.
ShA YD 123:14 the dregs, pith and seeds collected from non-Kosher wine are not Kosher but will become Kosher when they are removed from the wine and stand for at least 12 months. This is the opinion of Rebbi Yehudah in the Mishneh AZ 29b,