Yes, you may modify the KnowledgeTree source code. If you convey the resulting application to others you need to do this under the GPL and make all source code of the software (both the original code and your modifications) available under the terms of the GPL.

You would need to comply with the GPL's requirements around retaining the "Appropriate Legal Notices", both in the source code and on the user interface. In the case of KnowledgeTree Community Edition, these are the "Powered by KnowledgeTree" logo, the original copyright notices and any disclaimers. If for technological reasons you are unable to include the "Powered by KnowledgeTree" logo, you should replace it with "Powered by KnowledgeTree" text.

You will need to clearly indicate that the software is a modification of original code and the date the modification was created.