You can find the gradient tool in the Affinity Photo toolbar, you can select it and then create a linear, radial etc design using the current preset on a pixel layer etc, it needs a layer to work with (so an image layer or a layer created via new layer)
You can use it with all types of layers, images, pixel layers, shapes, etc By default, applying it which always seems to be the same default, black to white and linear (sadly, it doesn't recall the last used and type). That is the default, you can change the generated present quickly enough by changing the origin and length by dragging the line and re-positioning it.
By default, it always goes back to linear but there are a number of other types such as ellipse and radial and conical and bitmap (sadly missing a diamond or fractal etc). There is also no repeat feature. The type can be changed via the top bar. The ellipse is very similar to radial but the ratio between horizontal and vertical is not 1 and you can create elliptical shapes, you can also use it to create radial. It also includes a solid color type (a basic fill) as well as a bitmap pattern option which is superb for quickly importing a bitmap image and using as a tile
Next to the type along the control bar, you can click on the current preset and change the colors and stop information. The first two stops by default at 0 and 100% cannot be moved, you can change the color as well as the opacity stops but not their positions. You can always add new color stops to the editor by double clicking on the editor line and then changing the color and opacity in the same panel so the color can be set to green or red and the opacity set to 100% to transparent at 0%. You can also move the midpoint between the color stops by dragging and you will then get more of one color stop than another instead of half and half. You can add multiple color stops, you can also select the stop and press the delete to key to remove it. As soon as you leave it, it minimizes.
You can also rotate the preset interactively. You can also click the rotate button adjacent to the preset / editor. It rotates by 90 degrees to 180 to 270 etc, anti-clockwise based on the start position. This is the same for linear and conical and elliptical etc.
The initial application to a layer is black and white by default and always revets to linear. The reverse button just reverse this to apply the white / black. More useful is if you have a complex preset with multiple stops and the reverse will flip all the stops.
Unfortunately, only a single preset can be added each time, to create any number of combinations using blending modes requires the use of multiple layers. So on the first layer, create a design and then go to the layer menu and create a new one and then add another and then go to the layers panel and use the blending modes to blend perhaps with difference or darken or lighten etc.
Once you have applied the preset, it can be changed, you can quickly apply all kinds of effects to the design such as the deform filter or add noise filter etc
You can create a gradient and then select the layer and then go to the layer menu and create a new pattern layer from the current selection. You can then re-size the pattern design as well as rotate it though you should not that the gradient design is now frozen into the pattern layer, it is not using the design beneath as a proxy source. You can though combine the current pattern layer with the underlying layer and you can also change the underlying pixel layer and either create a new pattern layer as well as use blending modes to blend the pattern with the design. Once you are happy with your combo, you can merge visible into a single layer and then add effects such as deform filter to that and then use the pattern layer feature again if you wish to create all kinds of amazing artworks. At each stage, the design can be be exported as a JPG etc. You can also create new pattern layers by using a selection applied to the bounding box to only take part of the design into the new artwork tiles such as an arc if using an elliptical marquee etc
You can go to the layer menu and fade command to create many interesting designs by fading this and setting an opacity as well as a blending mode such as difference.
You can create a very quick rainbow preset by clicking on the editor and changing the stops and adding additional stops such as red at 0%, yellow at 20%, etc. You can set them at 20% 40% 60% 80% 100%. The 0 and 100% you cannot move. You can then change the color for each of those stops to blue, magenta, etc. You can then select the move with the filled shape and go to the swatches and add it to that for future use