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Ultimately, polyphase or otherwise, the sampling rate is likely too low to process a signal with a frequency at 1 GHz. Typically I use about 30% margin to allow for realizable analog filters. (With that $f_s > 2.5 GHz$ is recommended unless you have a high performance analog filter already selected that can satisfy a tighter requirement).

Ultimately, polyphase or otherwise, the sampling rate is likely too low to process a signal with a frequency at 1 GHz. Typically I use about 30% margin to allow for realizable analog filters. (With that $f_s > 2.5$ GHz is recommended unless you have a high performance analog filter already selected that can satisfy a tighter requirement).

Images and Aliasing in General

Images and Aliasing specific to OP’s implementation

Images and Aliasing in General

Aliasing in A/D Conversion

As for the process of aliasing that happens in an A/D conversion, please see DSP.SE #91854 where I have provided further details in explaining how that occurs. Because of this, an analog anti-alias filter is always required prior to the A/D Converter input, and as similar to the D/A consideration, the more margin we can allow in keeping our maximum signal away from Nyquist, the easier it will be to realize the necessary analog filter.

Images and Aliasing in General