Parallel execution, is it possible with the free version of katalon studio?

You can specify “parallel” mode for a Test Suite Collection. Katalon Studio free version provides it.

You want to create a Test Suite Collection which comprises with multiple Test Suites (TS1, TS2, TS3,…) that contain one or more Test Cases. If you specify “parallel”, then TS1, TS2, TS3 will be executed in parallel.