Depending on WGM160P part number chosen by customer, WGM160P features or not an integrated chip antenna.
The WGM160P is certified for FCC/ISED and CE with the integrated antenna (see section 4.7 of the datasheet) or with an external antenna (See section 11.1 of the datasheet https://www.silabs.com/documents/login/data-sheets/wgm160p-datasheet.pdf). The WGM160P certification ID is provided in the datasheet so that certificates can directly be downloaded from the web (See section 11 of the datasheet).
The external antenna used for certification on RF port is 50 ohms connectorized coaxial dipole antenna with maximum gain of 2.14dBi, achieving at least -10dB return loss over WiFi frequency band. An example of such external antenna, is the Pulse Part Number W1030.
Any external antenna of the same kind and with equal or less gain and at least same -10dB VSWR over the WiFi frequency band should be fine and should not void FCC/ISED certifications as long as spot-check testing is performed to verify that no performance changes compromising compliance have been introduced.
Any other antenna (such as a chip antenna, a PCB trace antenna or a patch) will require the customer to do some testing and apply for a C2PC (Class 2 Permissive Change) as indicated by the customer’s TCB (Telecommunication Certification Body).
With CE, anyway, customer has to perform certification test with the end-product.