I'm 18 but i've only gained 11kg since the start of puberty. i was very late bloomer and am now 6'1". barely any muscle with some fat (chest mostly)??

6'1": You should be seen by a hormone specialist and check whether your height is growing and to what extent. Getting taller without muscles is OK for 6'1" for a while but you do need to see someone who is expert at increasing your muscle size and strength. There are many places (physical training) that will help you gain weight and improve your strength.