Marjin buu saga: Edit
During the Marjin buu saga goten and trunks were being trained by Goku then later by Piccolo so the two trained and then he was finally made. Gotenks had incredible power and was very full of himself he was sure he could beat Marjin buu, Marjin buu easily beat Gotenks though so he came back and it was suggested by Piccolo to do it whilst being a super sayian.
Super buu Edit
Gotenks transformed even better this time as trunks and Goten were sayians so they knew they were good so they wanted to test out there strengths so being the kids they are they ran around the world a dozen times very quickly.