For your first attempt, the only issue is that tuus, -a, -um should agree with auxilium. The genitive pronoun tui is not usually used for possession, with some rare counterexamples. I would thus amend to:
Gratias propter auxilium tuum tibi ago.
As for your second sentence, I would advise against using the gerund in this way. The accusative of a gerund is almost always used with ad to express purpose, and it would be incorrect to use it alone in this way.
As a more general comment, the "two" sentences you are translating are semantically identical. It's an easy trap to fall into, thinking that help or helping need to be translated respectively by a noun or gerund. Especially with Latin translation, you need to focus on the sense of the sentence much more than the individual words.
As for your first question, propter is perfectly fine, though I tend to prefer ob as a shorter (and fairly well-attested) combination. One possibility:
Gratias ob auxilium tibi ago.
If you wish to emphasize the fact that the person was helping me, a more prolix version could be:
Ob auxilium mihi oblatum gratias tibi [plurimas] ago.
Which is literally:
Thank you [very much] for the help you have given me.