I have definitely used this exact thing, but I guess you need to do it directly within sageplot? That would be my guess as to your problem. Here is something I use which definitely works:
\sageplot[scale=.35]{plot(cos(x),(x,0,pi),fill=True,ticks = [[0,pi/2,pi],[-1,-1/2,0,1/2,1]],tick_formatter=[pi,None],fontsize=30)}

Actually, upon further reading, that is definitely the problem; you can't use show() within sagesilent because that returns None. The plot g1+g2 doesn't have any of those other attributes. You might be able to put them in the definitions of the plots themselves.