Cloonix

Hi @Cloonix 
To take the guess work out of matching up gradients use it on the group or layer.
That way each element inside will have the exact same gradient fill to start (as opposed to copy > paste style. That won't work here). Then just tweak the nodes on individual objects. What you don't touch will continue to match up perfectly.
So, in the example:
Made the master gradient on the parent Layer. 
Changed the top node (and added a second a little lower) just to the arrow.
Changed the shadows to black.
So.... the bottom two nodes never change and the transition from arrow to the D shape is seamless.
(oh, and as you can see, I treated the shadows as separate objects instead of using FXs)

Hi, Cloonix,
I came at it in a slightly different way.  I took a base shape, duplicated it a couple of times, and applied color gradients and transparency fades. It is a bit fussier than one might like to do.
DSweep.afdesign

Yeah, the hard part is perfectly matching the gradient for the patch. Making it no larger than is needed to cover the shadow cast by the arrowhead onto the shaft makes it somewhat easier. Getting the angle of the linear patch gradient the same as for the shaft is important -- I didn't really try to do that but one way would be to create a pair of temporary lines to use as a guide since I don't think there is any way to set the angle numerically.
As for creating the patch shape, I just drew it with the Pen Tool using snapping to set the size to the width of the shaft & its height to the intersection with the arrowhead shape, & then moved the two top nodes upward slightly one at a time, using the edges of the shaft below as a guide to make sure everything stayed aligned. It is harder to describe than it is to do, but if you experiment a bit, particularly at high zoom levels, I think you will get what I mean.

There is probably a better way to do this (& a better execution of this way), but what I did in this Ebene1.afdesign file was to cover the base of the arrowhead shape with a small patch & match (more or less) its gradient to the gradient of the surrounding area.

This Affinity Spotlight article has PDFs and souce files for the latest Affinity Designer and Affinity Photo Desktop keyboard shortcuts. 
https://affinityspotlight.com/article/downloadable-affinity-keyboard-shortcut-cheat-sheets/
 
It is part of the Learning category that compliment the video tutorials  

Hey guys......so I was working on my new course...and got the the lesson on making brushes.....sooooo I had to amek some brushes. 
 
I made a set of fog/ smoke brushes in raster and a set of silky smooth stroke brushes for your vector applications. 
 
I have learned so much from the people on the site....I wanted to give back, so here ya go . I am working on new blog post and some new art using them....so I will keep you in the loop 
 
Feedback on them is always appreciated, I love making these sorts of tools 
 
Jeremy , www.7thseasonstudios.com
Stroke brushes -7th season studios .afbrushes
Smoke and fog brushes -7th season studios.afbrushes

Try using menu "Layer/fill mode" and switch from the default "Alternate (even-odd)" to "Winding (non-zero).