Question: What is the ruling on food that contains what is called gel or beef gelatin?
Answer: If we know that the cow died by other than a human action or that the slaughterer did not slaughter the legislated slaughtering or it is from what is not permitted to slaughter, then it is dead meat, not permitted to eat it. So if a Zoroastrian slaughtered the cow then it is haram (forbidden), even if he mentioned Allah’s name and even if the blood flowed. If a Muslim slaughtered it, however upon a form other then the legislated form, then it is also haram and dead meat. So if something is taken from it and it is mixed with other than it and a trace from its taste, colour, and smell is apparent, then it is haram. If it vanishes in it [the mixture] and it is not apparent a trace then there is nothing wrong with it, because the Sahaba radiallahu anhum used to eat from the cheese of the Zoroastrians and the Zoroastrians’ slaughter is haram. However it is not taken from the rennet in cheese except a little which does not make apparent its trace in the food. So that proofs that something which does not make apparent its trace has no trace”.
[Taken from Liqaa al-Baab al-Maftooh no. 48 found here:
Faisal ibn Abdul Qaadir ibn Hassan