Transcribing DNA to mRNA with introns
I think the only place you are going wrong is in getting confused about strands ("take reverse complement").

5'-GGATCGTGCCACCATCCACCATCGTTA-3'   &lt;&lt; coding strand
3'-CCTAGCACGGTGGTAGGTGGTAGCAAT-5'   &lt;&lt; template strand


The coding strand has the same sequence as the transcribed RNA (apart from T>U), so the primary transcript is:

5'-GGAUCGUGCCACCAUCCACCAUCGUUA


Then, positions of introns:

5'-GG AUCGUGC CACCA UCCACCAU CGUUA


and after splicing:

5'-GGCACCACGUUA