"This is a fish."
It is always pronounced the same regardless of position. You may be perceiving it differently because English g is pronounced differently when it's not at the beginning of a word. Therefore, Gaelic and English g sound similar to one another at the beginning of a word, but different to one another elsewhere -- but it's the English g that's changing, not the Gaelic one.
I'm a total beginner, but I have a few things that might be related - please correct me if I'm wrong (which is likely)!
I think that "tha" cannot be used to connect two nouns together. "Tha" is only for connecting nouns to adjectives and maybe some other things. To say that two nouns are the same, "is" must be used instead. However, "is seo iasg" is not accepted either. Maybe "is" does not work with pronouns?