As a Roblox developer, it is currently impossible to simulate the pinch gesture in mobile device emulation:
https://developer.roblox.com/en-us/api-reference/event/UserInputService/TouchPinch
Currently, the input above is only possible to test in a real mobile device.
If Roblox is able to address this issue, it would improve my development experience because this will lessen the work to test, in addition to allowing you to debug the code inside Studio.