hsgamma using 85-95% of integrated GPU; 0% of dedicated

Please be aware that 30% of the time means run it full bore for 1 second then set it to idle for ~2.33 seconds.  This is likely to create thermal cycling (expansion and contraction effects) as the core heats up and cools down.  I'm no expert but that doesn't sound particularly good to me.  Apart from that, unless you have a really good cooling solution (100C doesn't suggest you have) there's quite a bit of danger in high intensity compute operations on laptops.

With BOINC, you either use a GPU or you don't.  You can't do the same throttling as you can with CPUs.

By telling BOINC not to use the internal GPU.

When BOINC starts, the event log shows BOINC detecting the GPUs.  By default, BOINC uses what it considers to be the 'best' GPU.  For some reason, BOINC must be thinking that the Intel GPU is 'best'.  You can configure BOINC to change that.  Read through the BOINC User Manual on Client Configuration for options like <ignore_intel_dev>N</ignore_intel_dev>, and others.  You need to create a cc_config.xml file.  There are full instructions in the documentation.  You just need to read it carefully and ask specific questions if anything is not clear.  I don't need this level of control so I don't have first hand experience with these options to turn specific GPUs on or off.