Drop tuning on a 34 scale lenght 5string

I don't think it's possible to install Hipshot GB7 Xtender on the SR375E, cause the tuning machines are configured as 3 bass side + 2 treble side. On this kind of configuration, possibly there's not enough space on the back of the headstock between the lowest string tuning machine and the neck volute for the Xtender mechanism.

Had it been other Ibanez SR 5 string with tuning machines on the headstock configured as 2 bass side + 3 treble side, there's possibility to use the GB7 Xtender, based on the following picture I got from googling and some older post on TB which showed a BTB 5er (which tuning machines were also configured as 2 bass + 3 treble), which had the Xtender installed, as it had enough space at the back of the headstock for it.

 

Yes it is possible.
For the A <-> C i suggest a .135 is optimum.
The higher string gauges depend on your tension preference, what gauges do you like for EADG?

 

DR has a string line specifically for drop tuning called DDT (Drop Down Tuning). DR makes great strings. You should try those for good stability with drop tunings.

 

DR DDT gauges are not suitable for your tunings.
First see my reply Drop tunings
DR DDT are gauged for all-fourths tunings, your unusual tunings are very different, containing a sixth in one tuning.
GCFA is tuned up from standard EADG by (mostly) 3 semitones, DR DDT do not have gauges that are suitable for that, the lightest they do is .100-.040.

As you are using .100 .080 .065 .045 for EADG, for GCFA i suggest .090 .070 .050 .030 for similar tensions.
So my suggested gauges are .135 .090 .070 .050 .030, which is an extra light 4 string set plus a single .135.

DR DDT are not a panacea for all possible low, drop or weird tunings.