By shorting an UltraShort I am in fact trying to get a long position in the underlying. I like the dynamic that as gold goes higher, my position in GLL decreases automatically. That's why I would prefer to get long gold in this manner.
Most of the Ultra ETFs have an inverse equivalent ETF where you can effectively short by getting long. I don't know about those ETFs in particular but make sure that they don't exist.
I believe the reason you can't borrow to short most of the leveraged ETFs is that people are shorting both the long and short versions in equal dollar amounts to capture the loss in value that occurs because of the large expenses involved in these ETFs. The 2X and 3X ETFs are only for day trading IMO.