It is easy and fast to switch from Gliffy to draw.io.
You mentioned that you have been using Gliffy for years, so I will assume you have lots of diagrams. Thankfully, the draw.io team built a really slick mass importer to convert all of your Gliffy diagrams to draw.io diagrams with only a couple of clicks.
If you would like to see the procedure yourself, there is a great guide here: Mass Import to draw.io from Gliffy & Mass Import from Gliffy to draw.io
Fun Fact #1: You can also import diagrams from Visio by drag and drop.
Fun Fact #2: Your deprecated Gliffy diagrams won’t be harmed in the import. If you need them back you can simply revert the page version in Confluence.