This is a pretty big question so please take a moment to think about it. A surprising number of people think that the best answer to this question is: "No." You shouldn't be able to trust your GM. If your GM is trustworthy then they won't keep you on your toes and you'll be able to become complacent. Sometimes this is because they mistake the question for referring to whether the GM runs easy games or hard ones, but it strikes far more deeply than that. Trust is about being able to let yourself become absorbed into the story rather than being defensive and trying to figure out what the GM is up to before they get up to it.
In a horror game, especially, you need to be able to trust your GM to scare or horrify you without leaving you frustrated or, worse, traumatised. A few nightmares at bedtime might be par for the course for some people and if they're okay with that then it's fine. Systematically rendering someone's PC helpless and then describing their awful rape scene while knowing that this is profoundly disturbing and upsetting to them ... is not. Helplessness in horror requires trust in your GM. If you don't have that trust then you can't generally get your players to engage in that helplessness without resistance.
It's also important to be able to trust that your GM isn't out to get you. They will introduce conflict and obstacles because without conflict there is no story. However the story is a collaborative effort that has a number of goals in mind. Very rarely do the players want to play a Zero-Sum game where the GM wins when the players lose because the players are far more restricted in how they play the game while the GM has access to an enormous toolbox and can throw just about anything at them. Sometimes players expect that GMs are treating it as a Zero-Sum game even when they're not either because of prior expectations from chatting to other gamers or treatment at the hands of another GM.
So with that in mind, consider the question. Consider how it affects your roleplay. If it's your GM's fault that you lack trust then perhaps you could broach the topic sensitively. Remember to use lots of "I" words. Odds are your GM simply believes that it's the way things are meant to be. They might revel in being devious and not realise that there's a difference between devious and cunning plots and boxing your PCs into a corner so you can pummel them into submission.
Or it might even be some small characteristic or trait that isn't commonly an issue but it occurs so often that you're starting to get frustrated and it's starting to damage that trust. Broach the topic but don't push it - especially if it's something minor. Often it's enough to simply draw your GM's attention to something and leave it at that. They might need time to think about it before changing their behaviour and if overly pushed they could become defensive and disregard your comments. Which isn't fair but it can happen.
If you're a GM, ask yourself if your players trust you. If not, why not? Talk to them about it. They might be a bit awkward about discussing their levels of trust with you - the person - so discuss with them any issues they have with the position of GM. Once you've distanced yourself from that role (because it might not even be about you but about them or their history instead), you can often both talk more readily about it.
If you're a player and you know you have prior issues, it could help to talk to your GM and let them know what your triggers are. You could explain, for example, that while you doubt your GM will have their GMPC take over things but that you've had such a terrible experience in the past that you don't think you can hack it just yet. Perhaps if you had a few months worth of sessions to build up to the idea?
Sometimes it might work just to let the GM know you're sensitive and then just monitor your own reactions. It could help just to take a deep breath and repeat this mantra: "This GM is not that GM. This GM is not out to get me. This GM is a good GM. I can trust this GM." Simply by drawing the line between your new decent GM and an old problematic one is all you need to do.
Anyway, have a think about it and tell me your opinions in the comments box below.