Multi-functional coatings derived from bio-inspired dopamine (DA) engage lots of attention for water environmental remediation recently. Nevertheless, high cost of DA monomers seriously hampers their broad applications. Herein, an elegant multi-functional coating which could be deployed as selective adsorbents or loose nanofiltration membranes for removing of dyes and antibiotics from water, were designed based on the Michael-addition and Schiff-base reactions between cost-effective plant-derived gallic acid (GA) and branched polyethyleneimine (PEI) under room temperature. Aside from the extremely low cost of GA (about 5% of DA), the resultant GA/PEI coatings possessed much better performance than DA/PEI coatings. When deployed as dye adsorbent, the bio-inspired GA/PEI coatings exhibited much higher selectivity, showing 40 % increment in anionic dye adsorptive capability and 30% decline in cationic dye adsorptive capability compared with DA/PEI coatings. When applied for nanofiltration membrane to separate antibiotics solution, the multi-functional coatings demonstrated solution flux as high as 41.3 L m-2 h-1 at 5 bar alongside 96.2 % azithromycin (AH) rejection, showing 20 % increment in flux and 10 % increment in rejection compared with DA/PEI nanofiltration membranes. Therefore, our cost-effective multi-functional GA/PEI coatings show promising applications for sustainable water remediation.