Is there a way to combine two gvim windows into one window with multiple tabs?

I will often open one file in gvim, go back to the terminal, and open another file in another directory in a new instance of gvim. I wanted to know if there was an easy way to combine these two instances of gvim into one window with each file in a separate tab, or if there was an easy way to open a second file in the first gvim window. I don't want to have to copy the path of the second file to use in the first gvim window, and I don't want to do file searching in the first gvim window.