Steps to reproduce:

set breakpoint to somewhere
run debug project
click "Insert new expression evaluator"
Enter "$xmm0"

A watcher for $xmm0 is now created. You can expand it to (theoretically) see the v16_int8, v2_double, v2_int64, ... interpretations of the register content, but the content itself is always "<not accessible>"

when I run the command "print $xmm0.v4_float" directly inside gdb window, I see the result as expected.

So maybe qt creator is not able to query gdb for the register contents?

Maybe the expand fuctionality should also be implemented inside the "Registers tab"