hey,

here i am suggesting some good books for GATE 2021 civil branch

1. for engineering mathematics - discrete mathematics and its applications by kenneth rosen  , higher engineering mathematics by B.S grewal , advanced engineering by erwin kreyszig

2. for structural engineering - theory of structure by CK Wang , S.rammamrutham , theory of elasticity by timoshenko and gere , theory of structure by wilbur and norris , strength of materials/mechanics of structure by gere and timshenko , E.popove , L.singer , B.C punamia rammamrutham

3. for geotechnical - soil mechanics by gopal ranjan and rao , venkat ramaiha , S.K garg , B.C punamia , VNS murthy

4. water resources - water resources by SK garg, water and waste water by SK garg 1 and 2 , fluid mechanics - RK bansal and fox and richard

5. transportation engineering - transportation by justo khanna

for more details kindly go through the link

https://engineering.careers360.com/articles/what-are-best-books-for-gate-civil-engineering

all the best!