does a process node need to be capable of N GHz to implement an N Gb/s SerDes, or are there tricks to avoid that?

Yes and no, the SerDes can operate at a higher speed than the regular digital logic (at the cost of more power and/or area typically). However, there are some intrinsic device parameters that need to be met in order for the SerDes to work. Primarily transistor ft i.e. unity-gain-frequency of a transistor short circuit current gain (cite: https://community.cadence.com/cadence_blogs_8/b/rf/posts/measuring-transistor-ft). How much slower than this frequency is required for a receiver is a function of the noise margin of the incoming signal since a certain amount of gain is required to amplify the incoming signal to a measurable level.

thanks