行数、列数が不均一なデータをRで読み込む ref: https://qiita.com/suimye/items/af3686990abc139c04eb

file0.txt

rl <- readLines(con="./c3.all.v5.0.symbols.gmt.txt")　#readlineによる行データの読み込み
gs_list <- list()　　#list準備　

for(i in 1:length(rl)){
    dl <- unlist(strsplit(rl[i], "\t"))　#strsplitでdelimitorとして\tで分割
    gs_list[[dl[1]]] <- dl[3:length(rl)]　#dl[1]とdl[2]には、motif名、urlなどの情報があるので除いておく
}

#1番のリストから遺伝子名を取り出す。
na.omit(unlist(gs_list[[1]]))

file1.txt

==========================出力=======================================
  [1] "MEF2C"     "ATP1B1"    "RORA"      "CITED2"    "APP"       "MAP3K4"   
  [7] "ATP2B4"    "GRIN2B"    "PHTF1"     "TLK1"      "PHOX2B"    "MYH2"     
 [13] "SKP2"      "ZHX3"      "TGIF1"     "LUC7L3"    "PPARGC1A"  "TSC22D4"  
 [19] "SPAG9"     "MAPK3"     "PDGFRB"    "EFNA5"     "RAB10"     "NEK6"     
 [25] "LOC148872" "EPHB2"     "TBC1D8B"   "DMD"       "EGFLAM"    "DYNC1I2"  
 [31] "MAML3"     "TRAM1"     "CDC42EP3"  "OLFM1"     "CDC42EP5"  "KLF12"    
 [37] "RSPO2"     "KLF14"     "NTRK3"     "EPHA7"     "SEMA6C"    "SFRP2"    
==========================出力=======================================