Is it possible to edit the gerber file to replace the curves with arcs in either of those packages? I’m guessing this is @smilliken 's problem, but he didn’t specify the shape of the board he is trying to render. The other thing I have thought of (but haven’t tried and don’t know if will work) is replacing the beizer curves in the outline path fed to Fritzing with arcs by manually editing the path to replace the beizer curves with an arc. If that is done it is possible (but as I said currently untested) that Fritzing may replace the thousands of points (which may be needed to render a beizer curve) with a gerber arc command (I am assuming there is a gerber arc command, as I know very little about gerber format!) @microMerlin did this on a path I created for someone a while back, if I can find that post I may try rendering my (beizer curve) version of the outline and @microMerlin 's arc version and see if the gerber output is different or not. That may be an easy (or at least easier, as manual path editing is fairly hard!) solution to this problem. The outline I have done to date have beizer curves as the Inkscape path editor uses them (at least how I use it, it is more than possible I am using it wrong!) to make curves with the default settings.
edit:
@smilliken Could you please upload the svg you are using to make the board outline? If I’m testing, I may as well test on something that will be useful to someone 
and as far as I know the one of the fixes worked for the other poster.
edit2:
Nope, that doesn’t look to work. I grabbed the 2 svgs from this post:
https://forum.fritzing.org/t/trouble-figuring-out-how-to-make-a-custom-pcb/
and ran them both through Fritzing. They are basically identical with the curves formed via thousands of an inch steps in x to make the curve
my svg
G04 MADE WITH FRITZING*
G04 WWW.FRITZING.ORG*
G04 DOUBLE SIDED*
G04 HOLES PLATED*
G04 CONTOUR ON CENTER OF CONTOUR VECTOR*
%ASAXBY*%
%FSLAX23Y23*%
%MOIN*%
%OFA0B0*%
%SFA1.0B1.0*%
%ADD10C,0.008*%
%LNCONTOUR*%
G90*
G70*
G54D10*
X225Y1575D02*
X226Y1575D01*
X227Y1575D01*
X228Y1575D01*
X229Y1575D01*
X230Y1575D01*
X231Y1575D01*
X232Y1575D01*
…
micromerlin’s svg
G04 MADE WITH FRITZING*
G04 WWW.FRITZING.ORG*
G04 DOUBLE SIDED*
G04 HOLES PLATED*
G04 CONTOUR ON CENTER OF CONTOUR VECTOR*
%ASAXBY*%
%FSLAX23Y23*%
%MOIN*%
%OFA0B0*%
%SFA1.0B1.0*%
%ADD10C,0.008*%
%LNCONTOUR*%
G90*
G70*
G54D10*
X145Y1575D02*
X146Y1575D01*
X147Y1575D01*
X148Y1575D01*
X149Y1575D01*
X150Y1575D01*
X151Y1575D01*
X152Y1575D01*
mine is a 69K file and micromerlin’s is 68K so near identical in size and basically identical in format. So it looks like arcs in the path aren’t a solution here. I expect a Fritzing code fix is the only thing that will fix this (assuming there is a gerber construct that creates arcs which I don’t know if there is!)
Peter