What Attributes go into ST Blocker, ST Rusher, and Gunner.
I see ratings for Kick Catching for returners, and stats for kickers, punters, holders, and snappers, but not these other positions...You can do a free agent search by these positions and get their rankings in these three above categories, but what ratings make up these position strengths?

You can go to the My Team->Edit Player Weights area and see what your coach has recommended - for blockers/coverage, the sim uses pass blocking skills on the line and run blocking skills during the return.

If that's accurate jdavid, it should be noted that by default ST Blockers does NOT include Run Blocking in its weighted attributes (only Pass Blocking).

ST Blocker is the OL on the punt/field goal units. The ST Rushers (the DL) will also be run blocking after the kick.

What about the Gunners? Is that purely tackling ability? Maybe some speed?

In that case, ST Rushers don't be default have any weight for Run Blocking, which should be applicable.

Pretty much, the gunners are actually all of the kickoff return (except the kicker and returner) and the gunners on both sides of the punt returns (the players that line up wide).

BTW, you can manually set these depth charts in the position overrides if you desire.