First up... you don't need a huge fee for BTG... the blocks are almost empty... 5 sats/byte should be more than enough to get a confirmation in next block

You can calculate your rough transaction size as: (#inputs * 148) + (#outputs * 34) + 10

That should be fairly close to your final transaction size. So in you case, you'll have one input and should be sending to one address, so your transaction size will be:

(1 * 148) + (1 * 34) + 10 = ~192 bytes... at 5 sats/byte -> 5 * 192 = 960 sats.... or 0.00000960 BTG

The error message you are getting sounds like you are trying to broadcast a transaction that has not been signed by the minimum number of private keys required for the multiSig. Are you sure that you signed your transaction with the 2nd private key??