If your MAGI ends up being high enough that any part of your Roth IRA contribution becomes an excess contribution, you'll resolve that by asking the IRA custodian either to recharacterize the contribution to be a traditional IRA contribution instead, possibly nondeductible, or to make an explicit return of contribution, not a regular distribution. If you obtain a return of contribution, any gain is taxable and potentially subject to an early-distribution penalty depending on your age. The gain is calculated over your entire account balance from the time of the contribution to the time of the return of contribution. The calculation does not depend on the performance of any particular investment in the IRA (unless you have only one investment).
If there are losses there will not be any taxable gain. You can't use the losses to offset any other income, the adjusted amount of the distribution will simply be less than the amount of contribution being returned.
CFR 1.408-11 describes the calculation in detail: https://www.law.cornell.edu/cfr/text/26/1.408-11
