If i write about my own indie game with few screenshots or even better start sort of the developer diary, will it be considered as advertisement?

As in posting them here in the forums? No idea but some folks here are against using the forums as a blogging tool. If you do so, you;re going to get negativity from at least some of the folks here.

Best bet would be to contact GoG directly. That would be here:

https://support.gog.com/hc

I ===>>>STRONGLY<<<=== suggest you give a whole lot more detail as to what you want to do. As in spell out specifically as to what you;re going to do.

Good luck