MTG Salvation

I like how this deck looks from the beginning, but there's some room for adjustment. first of all, yes I think fleshbag marauder will do more than abyssal gatekeeper, the gatekeeper requires a little more work to trigger (and when it does you need another creature to sack) while fleshbag marauder (and merciless executioner, also) will consistently just be recurring edict, or edict + bolt.
aven riftwatcher seems awesome here, nice pick.
I don't think the Gloves or the Slyblade are doing you tremendously good work here, however... they seem less focused toward your gameplan, and less effective because of it. Harsh sustenance is in a similar place, as it doesn't look like you'll often have a swarm or anything in play - and Orzhov charm can only return Soul warden, which limits it to two modes, neither of which seem worth the casting cost. Smother, or Go for the Throat, or any other removal spell would do here.
While I hear you on the trying to build within your collection, I'll make a few suggestions fully aware you might not have access to them, if that's okay? blood artist seems like he would do well here, as would something as simple as reassembling skeleton. On the pricier end, bloodchief ascension might be a good addition, as it provides a very real threat that you can trigger easily. But you needn't go pricey, really, because with athreos even cards like death cultist have the potential to make a large impact for a very small investment. gnawing zombie can also generate insane value. If you happen to have a kuon, ogre ascendant lying around he'll do a lot of work for your plan here. martyr of sands is of course ideal. nyx-fleece ram can be really annoying to deal with, even more so when they have to take a bolt for their troubles to keep it gone. Any new creature with exploit is probably worth considering if you have any lying around from drafts or packs - minister of pain has pretty good upside.

Hell, if you wanted to go FULL DEFENSIVE, you could run words of worship and rely on... like... scholar of athreos and stuff. that'd be funny.

Agreed re:Fleshbag Marauder. Gatekeeper is just too conditional. Also not in love with Slyblade.

Knight of Meadowgrain is a strict upgrade from Ajani's Sunstriker.

Did you want to make it more Soul Sister-y? Soul's Attendant and Suture Priest would be fine here.

Demonic Taskmaster is an odd inclusion. If you want to be able to get rid of your own guys easily, maybe something more like Attrition or Culling Dais.

Also agreed you could use some more solid removal. You could ditch Blood Scrivener to make some more room, that guy never seems to pan out.