These are 2 separate collection requirements. Reg B defers to Reg C if the loan is subject to Reg C. If the loan is not subject to Reg C then you must collect the GMI under the provisions of Reg B.
If the loan is for the purchase, or refinancing of the consumer's primary dwelling and secured by that dwelling then you must collect GMI under Reg B. Reg B does not exempt temporary loans with the exception of construction only loans.
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The opinions expressed are mine and they are not to be taken as legal advice.