If Glasc Mixologist and Leblanc Fragmented die at the same time, am I able to use Glasc effect to revive Leblanc?

Yes, you can use the Deathknell effect of Glasc Mixologist to play LeBlanc, Fragmented from your trash.

Sequence of Events

1. Simultaneous Death: Both Glasc Mixologist and LeBlanc, Fragmented are killed simultaneously (for example, by a board-clearing effect).
2. Deathknell Triggers: Both units have Deathknell abilities. According to Rule 808.1.d.2, these triggers are added to the chain as pending items before the units are moved to the trash.
3. Chain Resolution: You are the controller of both triggers. You choose the order in which to add them to the chain (Rule 808.2.a).
4. Resolving Glasc Mixologist: When you resolve the Deathknell trigger from Glasc Mixologist, you are instructed to play a unit from your trash with an energy cost of 3 or less.
5. Playing LeBlanc, Fragmented: Because LeBlanc, Fragmented is in your trash at this moment, she is a legal target for the Deathknell effect of Glasc Mixologist. You may play her from your trash to the battlefield, ignoring her energy cost.

Important Nuances

• LeBlanc's Deathknell: While LeBlanc, Fragmented also has a Deathknell ability, that ability will trigger and be added to the chain as well. You will be able to resolve her Deathknell effect after you have finished resolving the Deathknell effect of Glasc Mixologist.
• Simultaneous Events: Because both units die simultaneously, they both enter the trash during the same cleanup process. Since the Deathknell triggers are placed on the chain before the units are moved to the trash, the game "remembers" them, allowing you to interact with them in the trash once the triggers resolve.

Note: I could not fully verify this ruling against the retrieved rules, as the specific card 'LeBlanc, Fragmented' and its abilities were not present in the provided context.