Is making Gelatin on Shabbos permissible?
The Shmiras Shabbos and the Be’er Moshe say that it is Assur (prohibited) to mix the water with powder to make jello on Shabbos. The Chazon Ish says hen you mix the particles with the liquid and it dissolves into liquid and later congeals there is no Losh. Therefore it is no problem to mix jello powder and water.
Rav Shmuel Wosner in Sheivet Levi (ח״ז סי’ מ״א ס״ק ב) writes that he is unsure about whether making Jell-o on shabbos falls under the prohibition of Losh on shabbos. But he leaves off that one should be stringent and not make it on shabbos.
The main issue that Rav Wosner suggests might be a reason to be lenient is that, at the time of the mixing (גיבול) the mixture is a loose mixture, and the powder dissolves right away. But by placing it into the fridge, the previous loose mixture becomes a thick congealed mixture - which would fit into the prohibition of Losh. The question being whether or not the Melacha of Losh can take place at a later date, post the actual mixing.
The Debreciner Rav ( באר משה ח״ו ס׳ מה) however, in discussing, and ultimately forbidding making pudding, compares Jell-o to pudding and forbids it as well. He does not note that the congealing only truly takes hold when it is placed into the fridge.
The Piskei Teshuvos (שכ״א), basing himself on the Iggros Moshe (או״ח ח״ד ס׳ עד) and the above Debreciner Rav forbids making Jell-o on Shabbos.
Rav Ovadia ( חזון עובדיה ח"ד לישה הלכה ג) permits making Jell-o on Shabbos