Fortunately with Lazarus you can defer this decision indefinitely and just use whatever seems best at the moment ;-)
I personally have used GTK2 on Linux for a long time, until I recently upgraded some of my *buntu boxes to the 21.04 release and found that they are now very aggressively phasing out GTK2 support, removing theme support for GTK2, etc.
Fortunately the Qt5 widgetset has become very good in the meantime, I have upgraded from Lazarus 2.0 to the 2.2 branch earlier this year and at the same time I began gradually switching all my old projects over to Qt5. This is causing ZERO problems so far, exactly as one would expect with such an excellent versatile Toolkit that Lazarus is.