Gur is past tense of go?
It’s not technically a past tense (since it’s a conjunction, and conjunctions don’t have tenses), but this gur is only used before a regular verb in the past tense.
Could you say something like "cheap mé go raibh beoir cheannaítear agam"
That’s somewhat scrambled in Irish. Which English sentence are you trying to express in Irish?
you can't use 'go raibh' instead of 'gur' here?
The two consecutive verbs in go raibh cheannaigh mé don’t make sense.
Why "gur" and no "go"?
See my reply to deserttitan above.
Does "cheapaim" not make sense for "cheap mé" here?
In the present tense you do indeed have the option of the one-word form for the first person singular: ceapaim. But in the past tense, you always have to use the form "Verb + mé" for the first person. "Cheapaim" doesn't exist.
We use "cheapas" all the time in Munster. And, indeed, most verbs can have an "(e)as" ending instead verb+mé in the past tense
Cheap mé gur
cheannaigh mé beoir.
Almost rhymes, that. I say "almost" because I'm not sure of my pronunciation.
Different vowel sounds, and the slender r in beoir doesn't sound like the broad r in gur.