Gretopia lets you choose any currency you like to view prices and compare flights, but some airline and travel agent websites will only show their prices or allow payment in a limited number of currencies. If these flights are shown on Gretopia, you should be able to book them.
Please bear in mind that if you pay for a flight in a currency that’s different from the currency your bank or card issuer uses, the currency conversion will be done by your bank or credit card company. They may use a different exchange rate to Gretopia, and may charge commission. You should contact your bank if you want to find out about the exchange rate that will apply to you.