One of my machines has a usable GPU, the other has not, so I went to the separate preferences pages but couldn't find anything. When to suspend it, yes, but nothing about whether to use it at all.
Am I missing something?

---

Rgds
Peter.

Then I found https://boinc.berkeley.edu/wiki/Client_configuration, which has instructions on excluding a GPU. I pasted the text to avoid typos, and this is my cc_config.xml:
cc_config>
<exclude_gpu>
<url>http://asteroidsathome.net/boinc/</url>
<url>http://einstein.phys.uwm.edu/</url>
<url>https://lhcathome.cern.ch/lhcathome/</url>
<url>http://milkyway.cs.rpi.edu/milkyway/</url>
<url>https://universeathome.pl/universe/</url>
<url>http://www.cosmologyathome.org/</url>
</exclude_gpu>
</cc_config>

Guess what? stdoutdae.txt now shows this:
Unrecognized tag in cc_config.xml: <exclude_gpu>

How do I proceed?

---

Rgds
Peter.

Cosmology at home doesn't have a GPU application

Then I found https://boinc.berkeley.edu/wiki/Client_configuration, which has instructions on excluding a GPU. I pasted the text to avoid typos, and this is my cc_config.xml:
cc_config>
<options>
<exclude_gpu>
<url>http://asteroidsathome.net/boinc/</url>
<url>http://einstein.phys.uwm.edu/</url>
<url>https://lhcathome.cern.ch/lhcathome/</url>
<url>http://milkyway.cs.rpi.edu/milkyway/</url>
<url>https://universeathome.pl/universe/</url>
<url>http://www.cosmologyathome.org/</url>
</exclude_gpu>
</options>
</cc_config>

Guess what? stdoutdae.txt now shows this:
Unrecognized tag in cc_config.xml: <exclude_gpu>

How do I proceed?

I think you may need to add the word <options> so , above I have . see wot happens .
This is one I used some years ago on other project , other than that I don't know
<cc_config>
<options>
<use_all_gpus>1</use_all_gpus>
<exclude_gpu>
<url>https://boinc.thesonntags.com/collatz</url>
<device_num>0</device_num>
</exclude_gpu>
<exclude_gpu>
<url>http://moowrap.net/</url>
<device_num>1</device_num>
</exclude_gpu>
</options>
</cc_config>