Results of gallium nitride crystallization by ammonothermal method are presented. GaN crystals grown earlier by a HVPE method on an ammonothermal GaN substrate and an MOCVD-GaN/sapphire template were used as seeds. Structural and optical properties of the obtained materials are studied and compared. Large radius of curvature (>100 m) and low dislocation density (7×104 cm-2) can be reproduced in the ammonothermal method using an HVPE-GaN seed grown before on ammonothermal GaN. This proves that the use of HVPE-GaN grown on ammonothermal seeds allows to reproduce high crystallinity in a subsequent ammonothermal growth. It also demonstrates that a much more effective multiplication process of high quality GaN can be launched using a combination of the ammonothermal and HVPE methods.