From experience, I can tell you that that Gotogate price may possibly raise by the time you finish with the booking page, so the difference is more likely to be 100TL, not 200TL on a like for like basis (GTG for example will try and soak even Elte+ members for a seat choice fee for standard seats). As others have said, if one of 50 things happen you are likely to be bled by them for any changes that might need to be made, so it is false economy. At the moment TK is running a 1 way sale IST-HRG for $68 all in. I would not bother for the perhaps $6 difference.

As an aside, this is NOT what these OTA's should be used for (you get these from Skyscanner or some similar service), where you are saving almost nothing and opening yourself up to all sorts of issues. OTA's are only really worth it when there is either:

A) When the OTA's on occasion offer multiple airline combo's as a very attractive rate in a way that is impossible to book any other way. This could be offering code shared segments or whatever that are not available any other way.
B) A larger OTA is trying to dump contracted inventory at rates which are ridiculously cheaper than you can find anywhere else (what we used to call consolidator fares). This could mean, and it happens on occasion that you look everywhere for let's say a LH ticket Biz ticket from SOF-(FRA)-JFK and the cheapest you can find anywhere is $3200, however expedia might have it for $1400. It is not a scam, but Expedia dumping distressed inventory that they have bought off of LH at extremely reduced rates.