But I got a permission error. So I started my Lazarus using sudo lazarus-ide and rebuilt. No error anymore.
Don't build with sudo.
A new executable is built under your user dir ~/.lazarus if the original one is write protected.
"startlazarus" is clever enough to know which executable should be started.
That's the idea anyway. Does it work for you?
BTW, I did not know GTK3 bindings are good enough for building the whole IDE.
I must test it when I have time.