Garbage collection: set trigger value

Question

I can see the trigger value in the output of gc():

> gc()
           used  (Mb) gc trigger   (Mb)  max used  (Mb)
Ncells  1173182  62.7    2310631  123.5   1173535  62.7
Vcells 97391820 743.1  171477824 1308.3 121138100 924.3

But how do I change it?