PlantUML is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
PlantUML distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this library; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
PlantUML can occasionally display sponsored or advertising messages. Those messages are usually generated on welcome or error images and never on functional diagrams. See https://plantuml.com/professional if you want to remove them
Images (whatever their format : PNG, SVG, EPS...) generated by running PlantUML are owned by the author of their corresponding sources code (that is, their textual description in PlantUML language). Those images are not covered by the GPL license.
The generated images can then be used without any reference to the GPL license. It is not even necessary to stipulate that they have been generated with PlantUML, also this will be appreciate by PlantUML team.
There is an exception : if the textual description in PlantUML language is also covered by a license (like the GPL), then the generated images are logically covered by the very same license.