This tool, as the name clearly doesn't imply, is a block inserter designed to replace the dated BTSD. It is backwards compatible with BTSD and will upgrade your ROM when using the tool. The readme has full details on benefits. A GUI is not included in this version. If you want one download it from an older version of GPS. The GUI does not support the features of the current list and has not been kept up to date. Use the GUI at your own risk.
Simple blocks, for example Mario-only, can be created with Blockreator.
Also, this update moves the map16 feature from spawn_bounce_sprite from $02 to $03 which might break resources that use it.
Apparently the changes were already edited in gps 1.4.0 without me knowing so I'll leave them for now, and if it gets accepted like this I'll keep it that way.