I wasn't aware that Geany uses Gtk+3 but that's good.

Yeah GTK3 support in Geany is not yet released.

What's happen? the text windows is blank (or gray) when you open Geany?

Yes, the Scintilla area is blank (so, gray). The line with the caret becomes drawn as soon as the caret blinks, but the rest remains blank until it gets invalidated (scroll, areas under other windows, etc. -- just as you expect draws to happen). Attached is a screenshot right at startup (just after the cursor blinked).

Which version of Gtk are you using. I have 3.9.15 here.

3.8.6 (Debian Sid). Also note that I don't use a compositing WM (Metacity), if it may have any incidence.