What do you mean by "marginal trasher?"
Weaker than Remake, Chapel, Steward, etc.
Doctor, without overbuy, suffers from very rapidly diminishing returns. It is a terminal action that doesn't take long until it ends up trashing single coppers, if you are lucky. When you first play it (assuming you have 12 cards), you kill on something around 2 cards (depending on the information you have and the shuffle luck). It is nice in that these cards don't come out of your hand (great you can buy something pricier than with Steward), but you quickly transition to killing one or fewer cards. Also bad, the cards you most want to trash in the early game - the estates are the ones Doctor has the hardest time efficiently trashing.
This is not to say that Doctor is a bad buy without overbuy, but I find if I want Doctor, I often want to pay 4; particularly if I can open 4 and use him again on T3 with a short shuffle.
Gherald: Depends on how much you want the Silver. If Altar is out, you are generally better increasing the variance (lower odds of hitting $5, higher odds of hitting $6) and grabbing a Silver, particularly if you have terminal draw like Oracle or Smithy already in the mix. Likewise, Forge and Grand market make pretty good cases for getting the silver going sooner.
I mean let's consider what your $4 might be. Dead draw. In which case, your Ghall is 1/3rd or 1/4th dead. Envoy is particularly harsh on this for the obvious reason.
Strong trashing - Chapel, Masq, etc. Chapel is the obvious one here. If I want to keep the Necropolis, then I might be able to kill everything but the GHall in two chapel plays, but that requires really lucky draws. Most likely I have 3 or 4 plays of Chapel to kill everything regardless of getting Ghall. Silver may well get you going faster if the trashing works.
Attacks. If Militia is out, having a junk Hovel is less damaging, I have decent odds of that being one of the cards I'd be discarding regardless. Getting to $5 when you are discarding or getting junked typically makes Silver more valuable.
Gainers. Either I'm building to an engine faster (e.g. using Iw to set up village/smithy) or looking at an alt-VP rush/slog typically. If the former, then typically my components can overcome the card count and GHall goes back to the dead card problem. If I'm going for alt-VP, those are often exactly the times you want coppers and you will pitch the Hovel soon anyways.
Ghall is not a bad move, but it really depends on what sort of money distribution you want in the early game.