The US Civil War was certainly almost SOLELY about slavery. Just look at the Declaration of the Reasons for Secession by South Carolina the very first state to secede. They mention slavery 18 times and the entirety of the very first paragraph is about slavery. I see no other states rights enumerated in the Declaration. Going through the documents related to the secession of the other southern states there is really only ONE right they discuss and it’s slavery.
And the Gettysburg Address didn’t free slaves anywhere, it was the Emancipation Proclamation that did so (eleven months before the Gettysburg Address). The Proclamation was executed under the president’s war powers as a “military necessity” but he couldn’t extend that to Union states OR the three Confederate states under Union control at that time. Lincoln did immediately begin push for legislative action to begin the process that would end in the passage of the 13th Amendment the following year.