A study by Loznjak Svark et al. (2022) from Denmark measured the selenium content of seitan (among lots of other stuff). The seitan and all other "samples were purchased in the urban area of Copenhagen or online between October 2019 and February 2020."
The seitan was wheat protein. You can also make seitan from spelt. But this was "normal" wheat seitan. They tested eight samples of seitan (eight different brands, I think) and found the seitan had a selenium content (mean with range) of: 32 (11–57) µg/100 g.
Interestingly, the United States Department of Agriculture (USDA) FoodData Central database has one listing for "vital wheat gluten" (i.e., wheat seitan) that has a similar selenium level: 39.7 µg/100g ("Based on another form of the food or similar food;")
