When entering manual corrections to 1099-B with errors, how do enter a gain instead of a loss when there is no choice to do so

You do not enter the gain or loss. You enter the proceeds and the basis, then TurboTax calculates the gain or loss.

if this is a transaction where the proceeds and tax basis are reported to the IRS (types A and D) and that is what was entered, why is the gain/loss wrong?  if the proceeds are wrong this creates a problem for you because of IRS matching, best would be to contact the broker to correct this.  if tax basis is wrong use the adjustment column and the enter an adjustment code from 8949 instructions.

if the wash sale amount is incorrect here's what the form 8949 instructions say

. If you received a Form 1099-B
(or substitute statement) and the amount of nondeductible
wash sale loss shown in box 1g is incorrect, enter the
correct amount of the nondeductible loss as a positive
number in column (g). If the amount of the nondeductible
loss is less than the amount shown on Form 1099-B (or
substitute statement), attach a statement explaining the
difference. If no part of the loss is a nondeductible loss from
a wash sale transaction, enter -0- in column

types B and E - proceeds but not tax basis reported to the IRS should be handled the same way. However, since the IRS does not get cost basis you could short cut it by changing the tax basis. 

type C and F neither proceeds nor cost reported to the iRS you could just enter the correct proceeds and tax basis.