|Gedcom is nothing complicated, merely a common format that is used to transfer trees between different software programmes. Think of it like an adapter that enables you to plug your UK plug into a French socket in your holiday home in France.|
I have checked on Ancestry where I also have several tree's and you cannot combine them on their site either. However you can on their software package, 'Family Tree Maker'. You may be able to do it on other software packages as well (like Roots etc) but I only know FTM. So you could download each of your trees to a version of FTM on your home computer (it's the Backup process in TP). If you haven't got FTM, maybe someone you know can lend you a copy. Then link them in FTM on your computer, (I have the instructions if you want them but they are in the Help section of FTM) then upload as a brand new tree onto Tribal pages.
...OR, you can type everything in again.