As suggested in the comment above, applying ‘Gaussian Blur’ on your objects would give the merging effect but it will not give a smooth gradient effect.
Here is a result for both.
Now on the Left side, the object is blurred by using ‘Gaussian Blur’ and it is seen that the lines don’t merge nicely also if you will try to blur the object more than this, the object and colors will disappear.
Now, for the result shown on the right side using “Gradient effect” - you can generate it using gradient tool. Here are the steps you should follow.
1) Draw the shape on which you want to apply your gradient.
2) Select the shape layer from the ‘Layer’ panel.
3)Select the ‘Gradient tool’ from the ‘Tool’ panel or press shift + g till the tool is activated.
4) You can select one of the given preset and choose your colors.
5) You can adjust the nodes to let the colors merge properly and also you can adjust the angle for the gradient for a better result.
Follow these steps and you will be able to achieve any gradient effect over any shapes you want.