Need of Crystal (During UART Programming A Bare SI-Tiny-Gecko Chip)
For the load capacitors

https://community.nxp.com/thread/388856

In summary from the link

CL = Load capacitance (check the datasheet of your crystal)

Cstray = Stray capacitance, varies by layout but usually 2-5pF

2 load capacitors CX1 and CX2

CX1 = CX2 = 2(CL - Cstray)