Thanks Axzd !
bcftools version 1.15.1
I first bgzipped vcf files using command:
bgzip -c file1.vcf > file1.vcf.gz
Then I used following command for indexing:
tabix -C -h -p vcf file1.vcf.gz
Finally performed merging as follows:
bcftools merge -l input_file.txt -O v -o merged_snps.vcf
I replaced G with . but now got this error:
[W::bcf_hdr_check_sanity] PL should be declared as Number=G
Is it necessary to perform imputation for GBS data ?