Just to let you know, I recently installed Debian 10, downloaded the latest Lazarus installation files from lazarus-ide.org (deb packages) and found no unusual dependency problems. However, there can be a conflict if you installed the fpc version from the repo. I found that Lazarus requires fpc-laz that comes with the latest version (as separate packages), which means that any fpc package from the repo should not be installed. Other than that I found no issues with gtk. I expect the latest Lazarus to also install without issues on Debian 9 either with Gtk or Qt based desktop.
What you may be referring to as gtk 2.0 though are the dev (development) files, but there's a whole bunch of dev files (not just gtk) that will be installed as dependencies of the Lazarus package (not fpc).