How can I make a shape like this in GZDoom?

There is no need at all to use slopes in this situation, and having that there are no floors over floors in your image you can avoid also 3D floors. 

You can create that shape simply by making a series of circles one inside the other. After that you have to raise the floor of the more external circle and lower his ceiling as well, by a small amount. Given enough circles you can raise and lower their surfaces proportionally to how far they are from the initial circle, giving the impression of a ) shape. Having that they are circles the shape is copied on all the sides so your final shape will be ) (. Remember to delete the central circle in order to create a central body for your structure and to make the first circle impassable in order to block your players so that they can not climb your tower like a stair.

To do it with slopes, start by doing what Simomarchi said, but with fewer circles.

Then, divide each side of each circle into separate wedge-shaped sectors.

Make sure all of the lines of every circle are facing the same way, either all outward or all inward. All inward is easier, so let's do it that way.

Next, select all the lines of all the circles, except for the innermost circle, and use action 181 (slope), and slope the front side of the floors.

Optionally, and only if you are using UDMF, you might want to scale the flats in the sectors with the steepest slopes so that they don't look too stretched.

Alternatively, if it's just for show (i.e the player doesn't interact with it), you can just use a model. You can download simple free OBJ models quite easily and then you don't have to worry about building it yourself. 

18 hours ago, Bauul said:

Alternatively, if it's just for show (i.e the player doesn't interact with it), you can just use a model. You can download simple free OBJ models quite easily and then you don't have to worry about building it yourself. 

I'm curious as to how to implement OBJ into maps...

slopes with that height would stretch the flat textures too far. I personally would use a model, but if you want to keep it simple, you know, it's a lot easier to just make a cylinder instead. Create a smoke effect on top of it and most player would know what this is supposed to be

On 7/6/2019 at 1:44 PM, Bauul said:

.... you can just use a model.

Quote

I personally would use a model

Ditto. When dealing with complex shapes you could try using 3D constructs, slopes, and other GZDooM fetaures. However, using models makes things far simpler. In addition, models are scalable and can be used with different textures with relative ease. Rescaling and/or retexturing sector-based constructs can be a massive pain.

@DoomSpud: Refer to this thread on ZDooM forums.

1 hour ago, ([zen3001]) said:

slopes with that height would stretch the flat textures too far.

You can always just scale the texture on a given axis to undo the stretch effect, but it is rather labor intensive (and is almost impossible to maintain neat tiling across sectors using textures that have strong, obvious patterns).

On 7/6/2019 at 12:17 PM, Simomarchi said:

There is no need at all to use slopes in this situation, and having that there are no floors over floors in your image you can avoid also 3D floors. 

You can create that shape simply by making a series of circles one inside the other. After that you have to raise the floor of the more external circle and lower his ceiling as well, by a small amount. Given enough circles you can raise and lower their surfaces proportionally to how far they are from the initial circle, giving the impression of a ) shape. Having that they are circles the shape is copied on all the sides so your final shape will be ) (. Remember to delete the central circle in order to create a central body for your structure and to make the first circle impassable in order to block your players so that they can not climb your tower like a stair.

I think this is the best option for me. I want to use as few GZDoom features as possible.

On 7/12/2019 at 2:59 AM, DooM_RO said:

 

I think this is the best option for me. I want to use as few GZDoom features as possible.

You don't need to make it a goal to use as few features as possible.  You just need to avoid trying to use as many as possible just for the sake of having a lot of features.  If it would improve your map, like in this case, go ahead and use that feature.

I wholeheartedly agree with @Empyre.Use features selectively, and do it in a manner that achieves specific objectives. For example, don't make a low-gravity chamber just because it's a cool thing to do; build it because it integrates into the overall gameplay of your map. To extend this example, you could have a upper-level area that is blocked/inaccessible. The only way to get to the upper level is to find the low-gravity chamber and jump/float up to the upper level. You can even add another layer of challenge that introduces ACS scripting - the fans or the centrifuge for the low-gravity chamber are not working at first, meaning you can't jump/float to the upper level yet. But either "repairing' the malfunction, or turning on the power, etc. in another part of the map makes the gravity function available. In this way, you have integrated scripting and low-gravity functions into your gameplay, and (hopefully) have made your map more engaging.