Gradient mode getter/setter does not have thread-safe access preventing proper serialization/deserialization

Reproduction steps:
1. Open the attached "GradientsAndThreads" project
2. Open the "ReproScene" scene
3. Enter the Play mode
4. Observe the Console

Expected result: No error
Actual result: "UnityException: get_mode can only be called from the main thread." error

Reproducible with: 2019.4.39f1, 2020.3.35f1, 2021.3.3f1, 2022.1.2f1, 2022.2.0a15

Reproduced on: Windows 11, macOS 12.0.1 (Intel)