Today we faced the same error, it turned out that the project created by GGTS has a groovy 2.3 compiler level, but the GGTS plugin is supported only up to the groovy compiler level 2.1.
The fix was to install the groovy 2.3 compiler for groovy -eclipse.
The compiler can be found in the "language and support tooling" heading in the extension browser, which is opened by clicking the "IDE Extensions" button on the STS dashboard.
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