On XFCE, many apps support Size/Placement memory. However, not all apps support it.
If gThumb does not support placement memory, then launch Settings > Window Manager Tweaks > Placement
to activate Smart Placement options.
I am not sure if you mean by this that you slid the slider all the way to the left (smallest) but this is kind of one of those funny almost counter-intuitive things. In order to have all windows affected by Smart Placement, you would slide the slider to the Right - to the largest.
We seem to have come across a rare event then. To put this in perspective, I have been on the Zorin Forum for several years now and this is the very first time that a user reported the XFWM4 Smart Placement not work.
You may try reloading the desktop... But other than that, either gThumb offers zero placement support at all or - there is a bug in the installed application.
I am not familiar enough with gThumb as an app to really troubleshoot its code straight off. I can review it when I have some time:
gThumb is a Gnome App for Image viewing. You might consider an XFCE alternative that does respect placement. (I believe the default is Ristretto)
I removed Ristretto because not being able to change the keybindings bothered me, but it had the same issue. By default it's set to always open fullscreen, but after turning that off, it still always opens in the same size/position regardless of how it was last time.
(I think this is actually a common issue for Zorin. There are a few other people who've asked the same thing on this forum, and none of them got conclusive solutions. The terminal not remembering its position is also hard to miss)
I cannot think of any way that would set Zorin OS apart from Xubuntu or Mint XFCE. Zorin OS does not modify XFCE or XFWM4 files.
I do not use Ristretto, either. I do not even have it installed. But I can try installing and testing it.
I use a dual monitor setup. I create themes and icon sets - so having window positions cooperate with my workflow is pretty important to me.
And my terminal always remembers its position. I know this with certainty as there are times I need to move it out of the way.
If your terminal and Ristretto are both not remembering position; this suggests that there is something else interfering with Window Placement and with Smart Placement.
You might check your ~/.config directory.
You can backup a copy of the directory and then reboot - this will create a new copy of your config directory, restoring all configs to default.
This can be undone by restoring the backup copy.
mv ~/.config ~/.config-bk
Test window position memory.
If that fails:
I am not sure at that point. My understanding is that a re-opened window will open in the same dimensions and position as it was in when closed.
For XFCE Terminal, under Edit > Preferences > Appearance you can set Default Geometry to remember a consistent size (this is not related to position).
If you are experiencing the majority of apps, if not all apps, not remembering size and position; It may be a setting that we are not considering.
If you are running all at default and experiencing the issue, we could escalate this to see if the ZorinGroup is aware of any default setting that they create specific to Zorin OS Lite.
I am still looking into this, but this application can be used to set the Size and Position of any application you launch so that it launches at the same place and size each time.
If you would like to check it out, we can work on configuring it.