Be warned that GW2 uses a FOMO cash shop model where they put stuff on sale for a week, and then you can't buy it until it goes on sale again in 6 months or a year. You can follow
this thread on the official forums to see what goes on sale each day, and you can use
this API to see
some things that will go on sale in the future. You can also check a skin on the GW2 wiki and check the gem store section to see when it last went on sale, though the wiki has been malfunctioning for the past month. If there is a particular skin you want, you should be checking these regularly and snatch it up when you can.