"Can GECU withhold distribution of monies if the first beneficiary refuses to pay the loan out of the whole balance and prefers that the loan be paid out of the second beneficiary's half?"
The beneficiary is not obligated to pay the loan; the estate is responsible for the loan. If the first beneficiary happens to be the person who is administrator/executor of the estate, this person isn't terribly bright. The lender will not be coughing up the proceeds of the account without the loan being repaid, as it is evidently collateral for repayment of the loan.
"What if the first beneficiary doesn't sign? Does that mean that the second beneficiary never get's 50% of the distribution?"
You haven't said anything to indicate the daughter and wife are *co*-joint beneficiaries, but that's always possible. You use words like "first" and "second", which seem to indicate to me that the "second" person is only in the running if the first is dead.
If the wife is a co-beneficiary, she's free to address the nonpayment of the loan in the probate court case. (That said, you don't say from what funds or assets this loan would be repaid -- the end result may be the same/that no one gets the money from this account.)