Text editor with tabs and syntax highlighting? Gedit alternative?

Geany

Geany is a small and lightweight integrated development environment. It was developed to provide a small and fast IDE, which has only a few dependencies from other packages. It is using only the GTK2 toolkit and therefore you need only the GTK2 runtime libraries to run Geany.

As a long time Gedit user, I shifted to Geany last year as my main text editor, I found it the closest alternative.
I found Geany supports the features you require and comes with many more features and plugins available in Ubuntu than gedit.
I find the code completion, symbol browser, code folding and tree browser useful, as well as having a more power find and replace tool as well as the ability to configure launching of command line tools from the menu.

I'm going to suggest my favorite editor, vim. Plugin support means it can help with your Fortran code with a few additional plugins. Of course, vim has a steep learning curve, so if you prefer something simpler to learn to use, it's not a good option.

Another suggestion is to look at the Fortran community's ideas on suitable editors:

Somehow no one's mentioned it yet. Try emacs.

It's the perfect programmers editor. It does come built in with Fortran mode, which supports indentation, highlighting, etc. The editing shortcuts can take some getting used to if you are coming from most other editors (although this is not nearly as true as it is with vim). CUA mode can help here. I suggest running through the tutorial (when you first start it up, it will tell you how). In the long run, you'll find emacs to be far more powerful than any other editor. Nothing, except for vim, even comes close.