values = []

a = [('SRV', 'CLIENT'),('SRV1', 'CLIENT'),('SRV2', 'CLIENT')]
b = [('SRV', 'CLIENT'),('SRV1', 'CLIENT'),('SRV3', 'CLIENT')]

gap = []
for row in a:
    if row[0] not in b:
        gap.append(row)
        
print(gap)