GPL-3.0 is a deprecated SPDX license

👋 GPL-3.0 is deprecated by [SPDX](https://spdx.org/licenses/GPL-3.0), please consider using [GPL-3.0-only](https://spdx.org/licenses/GPL-3.0-only) or [GPL-3.0-or-later](https://spdx.org/licenses/GPL-3.0-or-later). Thanks!

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Need to update the license headers for the source files or just project readme to be more clear about the GPL license compatibility. 