Thanks for your quick reply, Prof. Guy!
I just find the grid.grab() function in grid package. I think it is what I am looking for.
library(genoPlotR)
data("three_genes")
plot_gene_map(dna_segs=dna_segs, comparisons=comparisons)
gb <- grid.grab()
class(gb)
[1] "gTree" "grob" "gDesc"
Cheers!