The gradient tool is found in the toolbar of Affinity Designer. Click on that and you will see the options appear along the control bar where you will see context, type, picker and rotate and reverse.
You need a 'pixel' layer. Go to the layer menu and add a new pixel layer and then add the current selected preset (probably black to white or something similar). Or if you have an existing pixel layer such as an opened image, add the preset. You can add it to multiple different layers and then use blending modes to combine them.
Set the color used via the top / control bar such as the fill, type etc as well as the stops by clicking on the picker / editor to change the stops, positions, colors etc. You can also use the control bar to reverse the current preset so if it is black and white then it will become white and black. You can also rotate it via the rotate button which means for a linear, it will rotate by 90 degrees and then 90 degrees and so on. Will do the same for radial but that makes no difference. You can also click the button to maintain aspect ratio though that is mainly of use for the elliptical.
A vector shape created using cog / diamond / call out etc can be also filled instantly with the current preset. Select the shape and then apply the current preset in linear / ellipse etc and then you can also manipulate the original and length etc of the fill. You can fill the shape such as the diamond with shapes pasted inside into the design as well as duplicated as well as combined with blending modes. All kinds of complex artwork can be generated using the layers but very basic designs can be created as well.
There is no preset drop down on the top control bar but you can always select a preset via the swatches panel and gradients category. Swatches panel can be found via the view menu and studio and swatches. Just select one and the colors will change to reflect this (it is a pity they don't add more but you can also add your own presets to this swatches panel.). You can again go to the presets picker / editor and change all the stops and colors to create a totally different design from the swatch
You can click the editor / picker and you will see the stops and colors etc and you can change them, you can select any of the stops and change the color. You can, in most cases, change the positions as well as add and remove them (other than the two end ones) and you can increase or decrease the opacity to add some transparency to your preset.
All kinds of color combinations can be made with the layers (the feature has no blending mode or opacity which seems to be a weird oversight) and you can do this by going to the layers panel and selecting the layers at the top of the panel and changing the blending mode settings
You can create a preset via the editor with a stop with an opacity less than 100% and with that, you can see more of the underlying layers
You can select the mask thumbnail (you can create it by selecting the layer and then going to the bottom of the layers panel and clicking the mask layer button) next to the layer image in the layers panel, you can add the current preset to that mask and then show more or less of the underlying image.
You can use the current colors in the preset but you can also go to the editor and edit them but you can change them also by adding a layer above the design (a shape layer with a blending mode) or even better, an adjustment layer via the layer menu and the adjustments can include black and white or HSL etc and you can change the color in all kinds of ways. You can also add it to the mask for the adjustment layer and more or less of the color of the design will be shown without being modified by the adjustment. The adjustment layer can be re-sized and rotated etc so not all of the image needs to be modified by the color effect
You can select some type then apply it to the type, use linear or elliptical etc to add the current preset to the text. If you deselect the text and then come back later and then reselect it, you can still work on the color and change the angle as well as the length as well as modify the applied stops to the artwork.
You can open an image and then apply a preset to that but you have to add it to a new layer and then blend as there is no blending mode for it. Create a new pixel layer via the layer menu and then use it to add a preset (such as a sky color) and then use the blending mode for the pixel layer to blend with the image.