Your International marriage certificates issued in Denmark and Gibraltar are legally recognised worldwide, but for some government or immigration purposes you may need the further optional maximum level of Legalisation available on any document.
The Apostille stamp is a verification of authenticity by the Ministry of Foreign Affairs, some countries outside of the EU, USA, Canada, Australia may require this, we always recommend that you check with the offices where you are using the certificate to be certain. In all cases we can help with this. It is likely to be required if you live in Germany, or if you are NON EU.