None, really. Both are abbreviations of slightly different expressions meaning 'you know?' There are a number of similar tags with the same meaning.
I am totally confused with this section - and I thought 'do you know' is dach chi'n gwybod/nabod---
'gwyddost ti' comes from the short-form (conjugated) present tense of gwybod.
Its question form (wyddost ti?) is used in very shortened form ('sti, 'sdi) as a tag question - you know? - in some parts of Wales (definitely in the north-west). Other variants are also common, such as wsti?, ch'mod? (from dych chi'n gwybod?) and t'mod? (from wyt ti'n gwybod?).
[gwybod is one of very few verbs to have a true conjugated present tense separate from its future tense in Modern Welsh. As far as I know, the only others are bod (very common) and adnabod (very unusual to see it these days). You will not usually hear it in colloquial Welsh except in set phrases such as:
- Pwy a ŵyr? - Who knows?
- Wyddoch chi beth...? - Do you know what...?
Others I have heard are am wn i (as far as I know) and dwn i'm / dwn i ddim (I don't know), which are both from the first-person form gwn.
Don't worry too much about this, you'll pick it up from speaking to native speakers who naturally add a sentence tag to many sentences.
Just concentrate on the main sentence structures.