how to convert fasta.gz file to .qza and/or to .qzv file format using online /webbased system...
Welcome to the forum!
Check out our tutorial for importing data into QIIME 2 - we have a section here that covers importing FASTA files.
In terms of using a web-based/online interface, you may be interested in using Galaxy - this is a web-based interface that has QIIME 2 as one of the available tools. Check out our announcement post here for more details on utilizing QIIME 2 through Galaxy. The instructions from our importing tutorial above won't line up exactly with what you see in Galaxy, since those instructions are for the command line interface - but the general commands will be the same.
Hope this helps! Cheers