Could be starvation - have you tried putting it in a bigger pot?

How big is the pot? Did you feed it in the spring? I ask as it may be pot-bound and/or not getting the nutrients it needs.

Sadly, your plant will die if you keep it in a pot - Eucalyptus gunnii wants to put down a very, very deep tap root into the ground, and if unable to do so, it will die, so that's likely what you're seeing the beginning of now. If you can find a place in the garden for it, plant it out, but be aware you need to prune it if you don't want it to get 55 feet very quickly - there are a couple of ways to do this, you can either chop it to the ground in March, when it will regrow and make a shrub up to about 6 feet by September, then repeat the process the following year and so on. Or keep it as a sort of lollipop tree, where you let the main stem get, say, 4 feet, then cut it off, let the foliage grow then keep it trimmed 3 or 4 times a year so it doesn't get taller than 7 feet. It's a bit of a task - this tree has strong apical domination, so there'll always be at least one shoot at the top reaching for the sky.

Not sure if this is how I say thank you for your comments, I will try a rpot or putting it in my garden which unfortunately is very small, hence it being in a pot. I really appreciate your responses and help. Thank you.

If you use the deepest pot you can find, it might last a bit longer...

Regular light feeding, and a hard prune every spring will help it to last longer in a pot, too.