The current DAP implementation doesn't support the breakpointLocations request. Breakpoint Locations request can be seen described in the spec here: https://microsoft.github.io/debug-adapter-protocol/specification#Requests_BreakpointLocations
This issue is resolved by this series: https://sourceware.org/pipermail/gdb-patches/2024-January/206148.html
I think it does support them, see gdb/python/lib/gdb/dap/locations.py But it doesn't support columns or the "stepInTargets" request.