gc ASCII: @ (sometimes works similarly, such at for 'at' as a preposition)
This small base shape in the right bottom of the frame is catching the following (the right side) characters,
and pointing the left direction. This indicates a preposition.
( 右側からつかんで左に働きかける補助的な小さな形)
. . . .

hand-sign:
Open the extended index and middle fingers diagonally inward. 
( 間をひらいた人差し指と中指を斜め内側に向けて伸ばす)

name: [di] di sound of dim ( ディ )

definition (grammar):

Base overlays with #gc :

To work like an English preposition adding the meaning of the other character in the base overlay.
The examples of the base overlay and the meaning are following.
(重ね相手の記号の意味を含んで前の語や述語を修飾する前置詞として働く。)

Related to a direction or a location 方向や位置にかかわって;

{gc,17(upper direction)} : above (touching or not does not the matter,
so sometimes it includes on also. (〜の上に(の)
(: the cloud above it) is the same to .
 (どちらも「その上の雲」)
Combining another character on top of this base overlay, , more detail of 'above' can be explained.

{+{08,17}(low)}: low above (〜のやや上の/に)
{+{07,17}(high)}: high above (〜のずっと高いところの/に)
{+{17,01}(the highest point)}: at the summit/top of (〜のてっぺんの/に)
{+17,{03,31(shape)}(diagonal)}: diagonally above (something) (〜の斜め上の/に)

{+18(down, under, lower direction)}: below (〜の下の/に)
{+19(inner direction)}: in, inside of (〜の中の/に)
{+20(outer direction)}: out, out side of (〜の外の/に)
{+61(front direction)}: in front of (〜の前の/に)
{+62(back, behind)}: behind, in the back of (〜の後ろの/に)
{+(01,28}(center)}: in the center of (〜の真中の/に)
{+{17,28}the upper surface}: on (the upper surface of) (〜の上面の/に)
{+{02,19}(the left)}: on the left of (〜の左側の/に)
{+{02,20}(the right)}: on the right of (〜の右側の/に)
{+{19,28}(the reverse side)} : in the reverse side of (〜の裏側の/に)
{+{01,48}(circumference)}: around (〜のまわりの/に)
{+{01,70}(halfway, amid)}: between or among (〜の間の/に) Related to relation and comparison関係・比較にかかわって:
{+21(the heading direction)}: to, for (〜へ/の)
{+22(the returning or original direction)} : from (〜から(の))
{+14(way)}: through (〜を通って/通じての))
{+{14,70}going over}: beyond (〜を超えて)
{+14(through, way),22(origin, cause)}: because of (〜の理由で/の)
{+{13,21}(situation of parallel)}: along (〜に沿った/て)
{+11(limited)}: until (〜まで/の))
{+46(work)}: by, by the work (function) of (〜による/よって)
{+45(relation)}: about, related to (〜について(の))
{+{28(surface),45(relation)}(touch)}: touching to (〜に接して/た)
{+05(not)}: without (no joining of the following word originally) (〜なしで(の))
{+06(opposite)}: in spite of (〜にもかかわらず)
{+13(equal)}: as (well) as, in the same way of (〜とおなじく)
{+{13,15}(similar: the same type of recognition)}: like..., in the similar way of (〜と同様に/の)
{+{07,08}degree, level: as how much or little}:
(when the following word has a shape), in the size of;
(when the following word has some particular level or quality), in the degree of (〜ほどに/の)
{+25(joined)}: with (〜で、〜とともに、〜つきの)
{+04(minus)}: lacking, without
(the following word was joining originally but not for this time) (〜を欠かして/た)
{+{13,07}(more)}: more than; this meaning flexibly changes according to what follows next;
e.g.: (when the following word relates to length): longer than, (in case relating to area): wider than, etc. (〜以上に/の)
{+{13,08}(less)}: less than (〜以下に/の)
{+{13,17}(upper)}: upper than (〜より上に/の)
{+{13,18}(lower)}: lower than (〜より下に/の)
{+{13,61}(further ahead)}: further ahead of (〜より前に/の)
{+{13,61}(further behind)}: further behind of (〜より後ろに/の)
{+{13,19}(inner)}: inner than (〜より内側に/の)
{+{13,{02,19}(left)}: farther left of (〜より左側に/の)
{gc (preposition),64(emphasize)}: even at/in.. (modifying front word with the following words)
(〜(続く語)にさえ)

Related to Place, Time or Condition (場所・時・条件などにかかわって)

{+10(place)}: at, in, on, where( as a relative adverb) (〜(場所)で/〜の)
{+{10(place),22(origin)}(the departure point)}: starting from (出発地が〜の)
{+{10,21(heading)}(destination, where about)}: bound for (〜行きの)
{+36(time)}: at (for 'time'), when (as a relative adverb) (〜のときに/の)
{+{36,61} later)}: after (〜の(時間的に)後に/の)
{+36,62}(earlier)}: before (〜の(時間的に)前に/の)
{+13(same),36(time)}: at the same time as (〜と同時に/の)
{+{36,70}(time space in between)}: while, during (〜の(時)間に/の)
{+53(situation)}: as (the condition of ...) (preposition) (〜なふうで/に)
{+11limit, 21to}: until (〜までの/に)
{+ tentative} (a phrase): in case if (the phrase)(仮に〜なら 後続語は仮定の条件)
{ + (stress)} (a phrase): even if ... (...だったとしてさえ)
{+15(recognition}: as (being considered) (〜として(の))

More prepositions can be created freely, but simpler is better to see.

Compounding with verb-symbols:

changes these verb phrases into qualifiers similarly to various English ...ing forms: e.g.
, , , , , , ....

. . .