Gayatri mahamantra is the most powerful mantra and Can remove any ill effect,irrespective of planetary doshas It is used for purification of soul.Soul once purified, it becomes divatma and for such a soul has no planetay doshas.
Gayatri maha mantra japa is more than sufficient to ward of all evils including manglik dosha. Manglik dosha is an evil effect caused due to bad positioning of mars in the native chart. Gayarti is powerful mantra and can ward of these doshas.
In addition , u should pray lord ganesha - fast on tuesday, sankashti chaturthi, pray lord ganesh everyday, chant his sankatnashanam stotra - pranamya shirsa devam.... 21 times a day, chant mangal graha mantra regularly, do donate some amt for poor children.. these things will reduce ill effect of ur mangal dosh.