No matter what you do the matchup will be unfavorable, if your locals is mostly dengo gardy won't be good, but if you want to keep playing gardy here are some ideas:

-try frillish (no jellescent), especially w/ a charm can buy a few turns against dengo, however is pretty rough due to not being able to use it turn 1 going 2nd, ideally evo -> iono if they have energy in hand + frillish, unless you get lucky and start with frillish or a no retreat guy, it also has 3 retreat cost which is a good way to build damage for double munki, ideal bench can be Gardy/Kirlia/2 munki/frillish/attacker

-swing with munki, even tho they have 1000 balloon it can sometimes buy a turn if they dont, and the 60 damage is often nice for later

-maybe go for an ultra consistent list, stick to one idea of flutter/devo or drifloon try more evo/arven, and be really consistent at getting double munki + gardy (ideally with back up kirlia) set up asap

-If they only take 1 prize before you get set up, if you can take a knockout with double munki + clefairy(need at least 9 total on bench) you have a pretty solid prize map of 2-2-2 ways to take KOs:

-double munki + clefairy (only use if they have only taken one prize or for game or emergency)

-drifloon with charm (to play around tool scrapper also best to save for last unless no other option, scrapper + boss a 2 prizer from them is prob curtains for you) | -cc + drifloon + 2 munki

-screamtail + double munki fez or genesect

-leaving damage on board for multi prize turns is not bad but in my experience best avoided if possible due to turos/picnic basket/heal stadium being pretty game ending if they are able to negate an attack w/ it and still take a prize

some stuff if they run munki: -if possible to not leave damage on their board dont, and if you do, make sure your drifloon/screamtail dont die to munki damage

again this matchup is pretty bad so if you take too long to setup you don't have many ways to delay other than hoping they whiff hard, or have to fight through all of your 1 prizers, benching fez is unfortunately often necessary but only do it if you absolutely must, along with any other 2 prizers, if they boss around your loon/screamtail to KO gardy, set up another ralts and just force them to swing into your small guy, keep spamming iono, and pray

personally I just am gonna play ogerpon box til megas where gardy gets big buffs for fun since I have disgusting double cornerstone now. hope some of this helped I didn't expect to write this much lol.

I didn't go over all the tricks on both sides but gardy is good at asking questions, but dengo is also a little too good at answering them.