First of all, i had to do a Google search to know what Gyaras (ekAdashi) actually is. :D
I am using KAmakoti website as my reference here:
Why rice is prohibited on this day even who has not taken fast?
It is because on the ekAdashi day the pApa purusha ( the personification of all sins) takes shelter in the grains.
yAni kAni cha pApAni brahmahatyAdikAni cha | annamAshritya tiShThanti
saMprApte harivAsare | aghaM sa kevalaM bhu~Nkte yo bhu~Nkte
harivAsare || [viShNupurANe]
Greatest of sins like brahmahatyA are present in anna or food on the
eleventh day of every pakSha. One who eats on this day, ingests these
sins and attains great misery.
Another reference from Vishnudharmottara PurAna (a minor PurAna) which condemns the act of eating on this day:
ekAdashyAM hi bhu~njAno viShNulokAcchyuto bhavet ||
One who eats on the day of ekAdashI is forever banished from vaikuNTha
i.e. loses eligibility to attain viShNuloka.
What not to do during (ekAdashi) fasting?
upAvR^itasya pApebhyo yastu vAso guNaiH saha | upavAsaH sa vij~neyaH
upavAsa involves getting rid of pApa karma and pApa chintana and
embracing good qualities, at least during the period of the vrata, and
renouncing all luxuries including the food.
gandhAla~NkAravastrANi puShpamAlAnulepanam |
Perfumes, jewelry, luxurious clothing, garlands, anointments,
collyrium, abhyanga, tAmbUla charvaNa, brushing the teeth, are all
considered to be varjya (to be avoided) during upavAsa.
The same is stated in harivamsha:
a~njanaM rochanamchApi gandhAn sumanasastathA | puNyake chopavAse cha
nityameva vivarjayet ||
gAtrAbhya`NgaM shirobhya~NgaM tAmbUlaM chAnulepanam | vratastho
varjayet sarvaM yacchAnyadbalarAgakR^it ||
Some further not-to-dos which are however the same for all fasting days:
It is interesting to note that brushing the teeth during upavAsa or
vrata is banned by several other authorities as well. vAchaspati
mishra quotes the following pramANa:
upavAse tathA shrAddhe na khAdeddantadhAvanam | dantAnAM
kAShThasaMyogo dahatyAsaptamaM kulam ||
Alternate authorities offer reconciliation by suggesting that the ban
here is on danta kAShTha saMyoga and not really on cleaning the teeth!
Hence cleaning the teeth with leaves etc without the use of kAShTha is
considered acceptable. Now, it is up to an individual if one decides
to give up the tooth brush and stick to flossing on the day of
nR^itya, gIta etc are also considered as luxuries and banned during
upavAsa, unless it is a part of pUjA and offered to the Lord.
Sexual activity, gambling and sleeping during the day are also
prohibited during upavAsa:
upavAsaH praNashyeta divAsvApAkShamaithunaiH || [devalaH]
Specifically during ekAdashI, maithuna is strictly prohibited. One who
engages in maithuna during ekAdashI puts seven generations of his in
upavAse tathA yaunaM hanti saptakulAni vai ||