use for loop or parallel:

$ find ./ -name "*.fq.gz"  \
    | parallel --dry-run 'seqkit grep -P -s -p AGAACATCTCTCGGAGCCACAGG {} -o {.}.filtered.gz'
seqkit grep -P -s -p AGAACATCTCTCGGAGCCACAGG ./G1_1.fq.gz -o ./G1_1.fq.filtered.gz
seqkit grep -P -s -p AGAACATCTCTCGGAGCCACAGG ./G1_2.fq.gz -o ./G1_2.fq.filtered.gz

You need a loop that goes through all the files in your folder. There are many ways to do it. One option is to go to the folder where you have your files, and run the following:

for f in *.fastq.gz; do
    name=${f%%.*} #Extract the name of the file without the extension
    zcat $f \
    | paste - - - - \
    | awk -v FS="\t" -v OFS="\n" '$2 ~ "TGGACTAC" {print $1, $2, $3, $4}' \
    | gzip > $name.filtered.fq.gz
done

To complete the set of tools I will mention using bbduk.sh in filter mode. It is part of BBTools package. I will also leave the loop aspect out since it is covered by others.

bbduk.sh -Xmx2g in=file.fq.gz outm=matched.fq.gz literal=sequence_you_want

You can turn off reverse complement lookup if you don't want that by using rcomp=f option.