Isotope licenses
Isotope has three kinds of licenses: open-source, commercial, and OEM.

Open source license
The open source license is designed for you to use Isotope to build open source and personal projects. The Isotope open source license is GPLv3. The GPLv3 has many terms, but the most important is how it is sticky when you distribute your work publicly. From the GPL FAQ:

If you release the modified version to the public in some way, the GPL requires you to make the modified source code available to the program's users, under the GPL.

Releasing your project that uses Isotope under the GPLv3, in turn, requires your project to be licensed under the GPLv3. If you are okay with this, feel free to use Isotope under the GPLv3, without purchasing a commercial license.