Far as I'm aware you can't limit the use of the dGPU with software as the OS will ultimately override. The developer of gfxCardStatus openly stated this many years ago before ceasing work on the project, disabling power switching of the dGPU in the app at Rev 2.3. gSwitch suffers the same issues as far as I'm aware with gSwitch not being effective on post 2011 MBP's, not that It matters much as the GPU's will still switch.
If the dGPU is done then the options are limited. It may be possible to replace the dGPU, although this is very specialist work. There is a post here on MR MBP how to disable the dGPU on some models, equally it not so straight forward, nor for the faint of heart.
One of my old 15" MBP's came back into my hands around 18 months ago and sat dead & idle until recently, with time on my hands. It's one of the infamous 2011's, equally this one to all intents and purposes is a trooper. After close to nine years of abuse with both the software & hardware cleaned up it's running well.
TBH I don't think it's ever run better, previously it would hit 103C (217F) with the CPU dialling back to 24GHz, now it holds a solid 3.1GHz and sub 100C (212F). Likely the benefits firmware and OS X updates (10.12) and still remains to be totally stock spinning HD and all.
Unsure what I'll do with it, equally it's seen off a good few MBA's and newer MBP's that have either failed completely or are very close to being junked, even the keys remain to be matt...
With this one I'll remain to run gfxCardStatus which will lessen the GPU switching to a point and avoid applications that needlessly call for the dGPU.
Q-6