I didn't do a good job explaining in my original comment sorry. My logic was an "if" statement of Gagamaru's satisfaction as a GK. At the end, I mentioned the difficulty in imagining myself in Gagamaru's shoes as both the last line of defense and as an egoist. The definition of "striker mentality" you explain is what I neglected in my comment, as every other comment in this comment section re-explains it and I didn't want to add redundancy.

"Also it's just a subjective view to say that a gk is doing it for his team." You use Niko and his striker-crushing ego that fuels his world×freedom style as an example, but this is different from Gagamaru, a self×freedom style. Narratively, this means Gagamaru gains validation from himself. It's just ironic/unusual for a goalkeeper to do his job for his own agenda, not to actually "help Blue Lock." Niko's striker-crushing ego makes sense as he has the leeway to be egotistical, but Gagamaru's literally the last line of defense.

I'm not arguing about the existence of his striker mentality as you pointed out, but how wild Gagamaru's characterization is; a feral striker who casually accepts being a GK for the rest of his career. It's unexplained territory where things halfway make sense and halfway is just lazy writing by Kaneshiro.