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I'd agree with @Bob_Zimmerman that ideally swap space used on a gateway should be zero, which means the gateway has no need for paging and swapping at all which maximizes efficiency.  However there can be transient situations (such as a policy install & connection rematch) that consume enough memory for temporary allocation of swap space to occur.   But in my experience once some swap space is allocated, Gaia/Linux doesn't release it until reboot even if there is now plenty of available memory.  This can be easily seen with commands such as free -m. 

My own personal rule of thumb is that if allocated swap space is more than 5% of total memory (i.e. for 32GB total RAM swap space allocated is 1.6GB) you should probably get some more RAM for your gateway unless a memory leak is present, in which case eventually no amount of RAM will save you.  sk35496: Memory leak detection procedure for a Security Gateway with Gaia OS