How to convert txt.gz into fastq.gz? I'm a newbie to bioinfo. I'm trying to use galaxy.networkanalyst.ca for the analysis of RNA-Seq data and for that I need to convert txt.gz files into fastq.gz files.

Link: GSM4273445 is one sample and the whole series is from here.
I've Googled to find out if there are online tools to convert it, but there aren't any. Besides, there is no discussion addressing this format conversion on researchgate.
I intend to use this RNA-Seq data and investigate the relative expression of some specific transcripts of interest. I have been introduced to 'galaxy.networkanalyst.ca' where the very first step is to upload fastq files. In addition to this I was aware of ncbi GEOdatasets as source of open RnA-Seq data.
Can anyone suggest a method or tool for this?
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.