How to convert txt.gz into fastq.gz?
I've had a quick look at the file you linked to and this is a featureCounts file - this list the number of reads for each gene/transcript (COUTNS.txt in the file name pretty much gave it away that it was going to be a file of, well counts).
I can't help you with Galaxy, but if you want to do an RNA-seq analysis, the count data + metadata is likely all you'll need.
If you really want to do the preprocessing and mapping yourself, the fastq files can be downloaded from the SRA archive (SRP242087) - click on the SRA link in your whole series link.