You can cut 1 inch pieces of 1/2 inch thinwall PVC, flare one end(I use an old plum bob for the flaring)so it will not fall through the 7/8th hole drilled in the gutter top.
When the plant is small, a cottom ball can be used to help hold it. If the root should try to grow larger that the inside diameter of the PVC, it will adapt.
If the top is not stable enough to hold the PVC, you could try using downspouts instead of gutter.

Ive used disposable plastic cups with the bottom cut out. Many sizes to choose from. Put two together and it is pretty sturdy. Also has a taper from top down so no flaring needed.