It seems to me you’re missing half the story… you’re asking about the fall, but I think BTG has great potential to rise. Also, to be honest, I think it has less potential to fall compared to BCH.
Think about this: despite the backing of industry behemoths, BCH has not managed to really encroach on BTC’s dominance. Despite creating a higher block size limit, actual BCH block sizes have remained much, much smaller than BTC’s. BTC has blocks 10x as large as BCH (before including Segwit), and has 10x as many transactions in them as BCH.
Although BCH was promoted as the “real Bitcoin,” one year later, it’s clear that Bitcoin is BTC, and Bitcoin Cash is not - based on price, based on volume, based on actual transactions per block, by all measures, BCH has not taken over as “BTC.”
If there’s no increase in adoption despite all the promotion and the weight of industry titans behind it, why would it rise? I would be concerned about seeing BCH price fall. At some point, most people will realize it’s simply not going to replace BTC. And with the exponential growth we’re seeing in Lightning Network, the kind of layer-2 technology all the knowledgeable computers scientists agree is necessary to handle really dramatic increases in transaction volume, BTC is capable of expanding dramatically. Soon, everyone will begin to realize that.
The “weak hands” holding BCH may well start selling off at some point, and once it begins, it may be quite hard to stop.
Meanwhile, BTG? Well, BTG has already suffered a big decline in price during this bear market. And, let’s be honest, there were plenty of detractors of BTG since the beginning - but many of them already sold off their BTG. I suspect we don’t have as many “weak hands” remaining that may dump.
BTG has also suffered because of the recent attacks on exchanges , and usability has been hampered because many exchanges have been (understandably) slow to resume normal BTG activity, even after our successful Network Upgrade on July 3d made those kinds of attacks impossible to repeat. We’re beginning to see rapid movement, now. Bittrex has re-opened wallets; Binance has dropped their confirmation requirements back to reasonable levels, as has Changelly, and others are doing the same.
And let’s not forget that the Network Upgrade also thwarted the ASIC miners that were trying to encroach on BTG; we’ve held firm on our promise to keep GPU miners in business, and the ability to meet this commitment to the GPU mining community has been met with accolades. Our community is strong and getting stronger, and we’re building valuable relationships with other players in the ecosystem - including other coin teams. We’re working hard to live up to our claim of being a “friendly fork.”
So BTG is a value from all those perspectives. In addition, BTG has never tried to screw with the coin supply, nobody is burning BTG, etc. - the economics are stable and traditional, just like BTC. Our inflation rate is very small, like BTC, unlike several other popular coins with inflation rates ten times highers. That confers long-term stability; it’s a “sound money” monetary policy.
And BTG is ramping up Lightning Network, which will grant the massive transaction speeds and scales that BTC is gaining, and it maintains interoperability with BTC. We’ll soon have cross-chain atomic swaps among BTC, LTC, BTG - while BCH is choosing to stay out of this arena.
And, given that BTG is at a low price, it has tremendous potential to rise.
To me, BCH looks like a failed takeover of Bitcoin. BTC is still BTC. Will BCH succeed as a stand-alone? Maybe. But it also has potential to fall; it certainly doesn’t have the strong-hand holders that Bitcoin does, and isn’t getting the mainstream attention that Bitcoin is getting. The new ETFs people keep trying to approve are for BTC, not BCH.
Meanwhile, BTG is successfully being a partner to others in the space, is proving its ability to adapt and thrive, and we’re bringing GPU miners a coin in the traditional Bitcoin model which will be interoperable with Bitcoin-based technologies. This gives BTG tremendous potential to rise.
In my portfolio, I can’t give my ratio of BTG/BCH - because you can’t divide by zero. 
(Disclaimer: I hold BTC, BTG, and LTC, but no BCH.)