Easy way - just look at this:
https://www.serebii.net/pokearth/ Click on the routes and it should tell you what is where.
Look at Johto - New Bark Town is the town you start from, heading north-west to Cherrygrove and Violet City, before south to Azelea town which has the 2nd gym, which should take up that first couple of hours (if not more, depends if you race through or not - it not it'll be longer than 2, probably). Basically routes 29-33 and any nearby caves (Dark Cave, Union Cave, Sprout Tower, Ruins of Alph and Slowpoke Well as well). Also base of Route 46 is accessible very early on.
From memory - Pidgey, Sentret, Rattata, Geodude, Spearow, Catepie/Weedle and Ledyba/Spinarak (depends on versions - Catepie/Spinarak = SS, others = HG), Hoppip, Hoothoot, Zubat, Mareep, Bellsprout, Gastly, Unown, Onix, Dunsparce, Wooper, maybe Ekans (SS) are most/all of the ones available before the 1st gym, not counting your starter Pokemon.
Yep - Mareep is definitely possible before the gym leader.
Anyways, closing then -if anyone needs to know, there's the link there for reference.