Note that Gatwick is LGW.
GTW is Holesov Airport in Czechia.
Given that you are talking about travel 11 months in the future, be warned that there will likely be several schedule changes to the ticket you already hold. If you add another ticket to the equation at this stage, then you have two tickets that may experience multiple schedule changes, making your connection unviable. My advice? Wait until 2-3 months before travel to book your separate ticket. Or build in an overnight (or longer) stay in London. As oneworld membership is important, you could fly from LHR to DUB on BA instead.